I just read that Virtual PC is now a free download.  If you've never used it, Virtual PC allows you to setup and run other operating systems (Windows XP, 2000, 98, DOS, Server 2003, etc.) in a virtual environment on your existing PC.  This is all done with a big file called a virtual disk.  Virtual PC simulates all the hardware, and the guest operating system never knows the difference.  It can even share the host computer's CD-ROM and network connection so that you can install software and get connected to the Internet or local network.  Guest OS's will even show up as additional computers on your local network!  You can start, shutdown, suspend and resume your virtual environments at will.

One of the best features is being able to "undo" changes to your virtual environments.  When this feature is turned on, every change made to the guest OS is saved to a separate change file.  When you "undo" a Virtual PC, all the changes are completely blown away and it is as if those changes never happened at all.  This is ideal for testing application or Web installations, giving demos, or trying beta software, because you can always roll back to the exact same state to repeat your tests or if something goes wrong.  We all know that uninstalling an application doesn't always (and more likely never does) put your file system and registry back the way it was before you installed.