One can use multiple Operating Systems (OS) on one computer with the use of a variety of methods that lets one run XP on a Windows 7 platform...  Unless the architecture of your computer's motherboard/CPU is configured for 64 bit processing and you're attempting to run a 32 bit OS without a very complex work around that isn't worth the hassle involved, yet you can use it if your computer is a 32 bit machine and both OS's are 32 bit based also...

Allow me to elaborate further... But first, let's answer the question of why? Running multiple operating systems on one computer couples the strengths and capabilities of those OS's while requiring that you only buy one computer... Who hasn't been running one OS and wished that they could run a program that only runs under another? Who hasn't experienced an OS problem and wished they could boot into another system to resolve it?

In computer refurbishing, running multiple OS's allows you to combine the strengths of an existing Windows install with those of open source.... Case in point: Windows XP is aging but still has active forums, tons of how-to websites, great ease-of-use, thousands of applications, and drivers for every device.... Keeping it retains the original software license, the installed applications, and the existing drivers.... (Past articles have described how to secure and performance tune mature Windows systems.)

Add a Linux distribution and you gain all the benefits of open source software -- a free and currently supported operating system, a state-of-the-art OS that runs on older hardware than Windows 7 or Vista, security without anti-malware overhead, and tons of free applications. A well-chosen distro greatly expands the capabilities of an aging Windows computer...

Here are the ways to run multiple operating systems on a single system...

1.) Live CDs and Live DVDs allow you to easily boot and run alternate operating systems... Just download a bootable OS image, burn it to disk, set your computer to boot from the optical disc drive, and you're off and running... here's a link to explain what a "Live CD" is:

http://en.wikipedia.org/wiki/Live_CD

One big benefit to Live CDs is that you're guaranteed a malware-free OS... Some experts now recommend against doing your banking and finances online, or they say you should not use Windows if you do... Live CDs offer higher security for online banking...   

http://voices.washingtonpost.com/securityfix/2009/10/avoid_windows_malware_bank_on.html

Every time you boot from the Live CD you initialize a fresh uncontaminated system, because the read-only disc can not be altered by malware... (Of course the original Live CD has to be virus-free but I have yet to hear of such contamination.)

You don't have to alter anything on your existing hard disks to run a Live CD. No fiddling with partitions... You don't even have to mount your internal disk... Live CDs are a risk-free way to try out and play with as many different OS's as you're willing to burn CDs for...

Whether you'll be happy with the performance of running an OS from CD or DVD depends on the speed of your optical drive (CD/DVD Player/Recorder) and the OS you run... Most people are satisfied using current optical drives, even for full-sized Linux distributions that continually access the disc while running... If you're refurbishing an old computer and frequently run live CDs, check the speed of your optical drive and get a newer one if you need to... Drive speed has dramatically increased over the past decade...

Another trick is to use a distro that runs entirely from memory and never accesses the CD/DVD or hard disk after booting... Puppy Linux, for example, runs entirely in memory on systems having just 256 Mb or more... It performs well even on older computers... After booting you can use the CD drive for purposes other than running the OS... The only small drawback is the start-up time it takes for the CD or DVD to initially load the OS into memory...

2.) Live USBs are a variant on the Live CD/DVD concept... Here's what a "Live USB" is:

http://en.wikipedia.org/wiki/Live_USB

In this case you use a USB memory stick, flash drive, or USB external disk drive to boot and run the operating system...

Like live CD and DVDs, live USBs are portable... You can boot an OS once this way to try it out, or you can work this way regularly... USBs offer writeable, persistent storage... Memory sticks have no moving parts so they have better access times than optical discs...

The big downside to USB's is that many older systems won't boot from them... This is a BIOS limitation... Here's what a "BIOS" is:

http://en.wikipedia.org/wiki/BIOS

Circumvent it by using a boot manager that boots the computer from a device your BIOS supports, then immediately switches control to the USB device... PLoP is one free boot manager... Another potential downside to USB booting is shortened lifespans for USB memory, due to its constant use as the OS resident media... Here's the link for PloP:

http://www.plop.at/en/bootmanager/index.html

3.) Virtual Machines: Desktop virtualization has matured in the past five years...    http://en.wikipedia.org/wiki/Virtual_machine

Hardware technologies like Intel's VT-x:    http://en.wikipedia.org/wiki/X86_virtualization#Intel_Virtualization_Technology_for_x86_.28Intel_VT-x.29    

And AMD's AMD-V underlie and improve it... These were introduced in 2005 and 2006, respectively... In other words, this type of application software enable one to run a separate OS that the one that was originally installed installed, in a virtual environment...

This chart:   http://en.wikipedia.org/wiki/Comparison_of_platform_virtualization_software

lists and compares platform virtualization software... Oracle VM VirtualBox:    http://en.wikipedia.org/wiki/VirtualBox    ---------->

is probably the most popular free offering for personal use... Once owned by Sun Microsystems, the product was taken over by Oracle with their acquisition of Sun in January 2010...

VirtualBox installs under an existing host operating system, then creates one or more virtual machines in which various guest operating systems run... The possible host includes most popular operating systems, such as Windows 7, Vista, Windows XP, Linux, Mac OS X, Solaris, and some BSD versions... Guests include all the host systems plus even some lesser known contenders, like Haiku, OS/2 Warp, Syllable, SkyOS, ReactOS, and the rest of the Windows and BSD versions...

VirtualBox offers all the advantages of platform virtualization:    http://en.wikipedia.org/wiki/Hardware_virtualization  
These include the abilities to:

    a.)Securely run more than one OS at a time.
    b.)Flip between the systems as you like.
    c.)Start, stop, and pause the systems independently.
    d.)Communicate between systems through several mechanisms.
    e.)Dynamically allocate and switch processor and memory resources among the systems.
    f. )Make an image of a current system state for backup/restore by taking a snapshot.

VirtualBox does not require CPU-based virtualization support (VT-x or AMD-V)... But it does require the cumulative resources to run all the OS's you want to at one time...
Thus it may not be an option for some refurbished computers... If your computer supports it, virtualization is a great way to go. Get started with VirtualBox with tutorials from here:

http://www.freesoftwaremagazine.com/articles/using_virtualbox_to_run_ubuntu

And here:    https://www.virtualbox.org/wiki/User_HOWTOS    

Visit the official VirtualBox website here:    https://www.virtualbox.org/

4.) Co-Installing to Disk: If virtualization and Live CDs don't appeal, you can always go the traditional route and install more than one operating system to disk...

Assuming the computer already runs Windows and you want to co-install a Linux distro, the typical procedure is:

    a.)Boot the Live Linux CD for the product you want to install.
    b.)Shrink the Windows partition to create sufficient space for the Linux install.
    c.)Create a new Linux partition in the newly-released space.
    d.)Optionally create a swap partition for Linux virtual memory.
    e.)Install Linux into the new Linux partition.
    f.)Install a tool like GRUB or LILO to display an OS selection menu upon system start-up.
    g.)Reboot and verify the OS selection menu and the new Linux install.

Let's walk through the steps...

First you boot a Live CD or DVD for the Linux you want to install... This allows you to test and ensure the distro drives all your computer's devices and works the way you want -- before you install anything... It also creates a working environment from which you can perform the subsequent steps... Most distros provide the partition management and boot control tools you'll need... Ubuntu and Puppy Linux, for example, both offer Live CDs that bundle the GParted partition manager:

http://en.wikipedia.org/wiki/GParted

And the GRUB boot-selection tool:    http://en.wikipedia.org/wiki/GNU_GRUB

Assuming the computer came with a single disk with Windows installed, you'll probably need to shrink the Windows partition to make room for Linux... How much space Linux requires depends on the distribution... A small Linux like Puppy installs easily in a gigabyte or even 500 megabytes, while a full-sized Linux like Ubuntu typically requires anywhere from three to eight gigabytes...

Beyond the operating system partition, you may also want to create a swap partition... Here's a link under "Paging" describing what is a "Swap Partition":

http://en.wikipedia.org/wiki/Paging#cite_note-autogenerated1-20

Linux uses this disk space for extra memory (or virtual memory) if real memory runs out... Do you need a swap partition? Think of it this way... What is the maximum size of memory you'll require for the Linux operating system plus the maximum number of applications you'll have open at one time? If this amount is greater than the size of real memory, you need a swap partition to make up the difference... 

Since users run different distros and use their computers in different ways, it's not possible to devise an all purpose rule-of-thumb for sizing swap space... Running typical home and office applications, for example, I never use the swap if the computer has at least 1 gigabyte of RAM... Specialized applications change this recommendation... For example, when I do database testing and research, even computers with several gigabytes of RAM need swap space...

The next step to installing Linux on a Windows computer is to ensure you have some free disk space on which to install Linux... You may have to reduce the size of the Windows partition on a single-disk Windows system because, by default, many vendors still pre-install Windows to consume the entire disk...

It's wise to run a Windows Disk Check prior to shrinking the Windows partition to ensure file system integrity. ..Then use the GParted Linux tool to shrink the Windows partition... You do not need to run the Windows defragmentation utility prior to this operation... Contrary to what you might read elsewhere on the web... GParted successfully shrinks NTFS partitions regardless of whether they have been defragmented...

At this point you have a target partition ready into which to install Linux, and optionally, a swap partition... Now you can install Linux... Most Live Linuxes have an "install icon" on their desktop that you click to start the install process... Although I've discussed getting the partitions ready prior to running this install tool, many Linuxes (such as Ubuntu), bundle the partition management tool as a step within their standard install process... In other words, you can either manually do the partition management in advance, or do it while you run the Linux distro installer...

Here's an example of GParted in action... The example below shows what you can do with multiple operating systems even on an old refurbished test machine with two tiny 40G disks... This system has four operating systems installed on its primary  hard drive... This includes a 13.67 gigabyte NTFS partition with the original Windows XP SP3 install. Then there are Linux partitions for Ubuntu, Puppy, and Vector Linux... The single Swap partition of about 510 Mb services whichever Linux runs... A second 40G disk drive (not shown), contains three more Linux distros, three backup partitions, and a BSD variant... Quite a lot of action for an old computer!

The first line in the above display is a FAT32 partition on which all user data resides... Storing user data apart from any of the operating system partitions is wise because it segregates data from the operating systems... This makes it easy to identify and back up user data... Moreover, the user data remains unaffected regardless of what changes you make to any of the operating systems or their partitions... Should an "OS disaster" occur it's unlikely that data on its own partition will be affected...

It is unfortunate that many consumer computers by default still store user data on a single Windows partition that consumes the entire disk... This practice is a hold-over from the days when disks were smaller and it doesn't leverage the advantages of today's larger disks... With big SATA drives it makes much more sense to segregate data and OS partitions... I know some people who even separate out certain kinds of data files into their own partitions... For example, you might have a "multimedia partition" for photographs, music, or videos... This makes it easy to separately manage large data files... Or segregate your work from your play by creating an "office partition" for office suite documents... Easy partitioning makes it possible to organize your system in the way that works best for you...

When adding Linux to a Windows computer, you'll install a boot manager, such as GRUB or LILO... In Ubuntu and Puppy, this is an option in the last step in the install process... GRUB takes over the master boot record or MBR on the computer's disk and inserts code that displays a menu to select any of the resident operating systems when you start the computer...

Don't worry about your existing Windows being bootable... GRUB is very good at automatically detecting any operating systems already on your hard disk... It generates the necessary code to make them boot-time selections... So GRUB will find Windows and automatically place it in your new boot-time OS selection menu...

When the install is complete, reboot your computer and you'll see GRUB's menu with options to enter either Windows or Linux... Try both out to ensure everything works okay and you're done... Voila! Multiple OS's on your computer...

Once you've gotten everything working, you might want to make the boot-time OS selection menu more readable... If you used GRUB, just edit the file menu.lst in the /boot/grub folder... (Save the original menu.lst file first as a backup!) If your Linux distro uses the newer GRUB 2, well, there are many advantages to this new boot loader, but easy changes to the boot menu is not among them... See this excellent tutorial for help:

http://www.dedoimedo.com/computers/grub-2.html

It's too bad that GRUB 2 is a step backwards for ease of use, especially since it would have been easy for the product to use a menu.lst file if present and convert it to the required internal code...

If your computer has two physical disk drives, you can use the second drive for backing up the operating system and data partitions... Disk-to-disk backups are fast, easy, and convenient... They offer quicker backup and recovery than tape or writeable optical discs...

You can use disk cloning tools to copy one entire disk to another, or partition copy tools to copy individual partitions across disks...
Here's a list of free disk and partition cloning tools:   

http://www.thefreecountry.com/utilities/backupandimage.shtml

I use HDClone Free Edition:

http://www.thefreecountry.com/utilities/backupandimage.shtml

To easily copy an entire disk to a backup disk. Like many of the free tools, it does the job well. Buy the commercial upgrade and you get a faster copy plus additional features like data reconciliation, data rescue, and system restoration...

Applications Compatibility... By "applications compatibility" what  I mean is this... Can Linux run your Windows applications? Many people find that they have a particular Windows program they'd like to be able to run under Linux directly... If the vendor does not already offer a Linux version of the application, try installing Wine under Linux... Then you can install and run most Windows apps directly under Linux.... This list gives you the details on the over 16,000 Windows applications that have been proven to run under Linux using Wine:

https://www.winehq.org/

If you have really old DOS applications you might install DOSBox on Linux... Designed to run games it also runs many other applications from the DOS and early Windows eras...
How about the inverse case for applications compatibility: Can you run Linux applications under Windows? The general answer is no, not unless the vendor also supplies a Windows version of the application...

The Bottom Line is this: Running multiple operating systems on a single computer offers compelling benefits... It allows you to gain the strengths of two or more OS's while only acquiring and maintaining a single computer... For refurbished computers, multiple OS's combine the strengths of the original Windows system, its license, installed applications, and drivers, with the thousands of free applications and tools offered by open source systems... Linux brings free state-of-the-art, secure, supported software to aging Windows computers... It's vital to computer refurbishing...

Platform virtualization is the premier method for running multiple operating systems on current machines, along with Live CD/DVDs or Live USBs. For older systems, Live CDs and multiple OS installs are popular... Mainstream Linux distributions come with all the tools you need to co-install multiple operating systems... This includes free tools for partition management and OS boot selection... Linux also offers good Windows compatibility. This includes install co-existence and the ability to read, write, and manage Windows files. Office suite compatibility is situation-dependent. But if it's important to you there are ways to achieve it. Linux also runs many Windows applications using facilities like Wine...

Well that's enough for now... I hope someone can make sense of what I just finished posting.

Respectfully,
Henry