Virtualbox is a program that lets you run a virtual machine. It's free, and exceptionally useful.
Virtualbox allows you to install virtually everything from windows 2000, XP, Vista, 7 to Ubuntu and other linux distros, without changing your computer in any way.
You set up a file that represents a hd in the virtual machine, then choose a iso, or use a cd to install your OS.
If you have ever wondered what a linux dsitro looks like, how something functions or if you would like to run an os, like ubuntu but are stuck with windows, and don't like the idea of dual booting, this may be the best solution.
You can download it here: