Microsoft Virtual PC enables you to run another operating system inside your main system, for example you can run a Linux system inside your windows system.
Now since Microsoft took over the project they have removed all documentation pertaining to running Linux inside Virtual PC.
But it still works. In fact its very easy to install/setup and run and works with a lot of the distros I have tried.
If you want to try out running Linux on your windows box this is the perfect solution for you.
you can even create virtual Hard drives for each distro you install.
http://www.microsoft.com/windows/virtualpc/downloads/sp1.mspxThe download is around 20MB and takes no time to download and install.
Then grab a copy of Linux (if you want to use something quick to download try a small live CD version like Austrumi Linux, Damm Small Linux, or a version of Puppy Linux like Mean Puppy [ALL under 50MB] ).
Capture the ISO and run.
Voilla, your running Linux in your Windows system.
Enjoy.