Virtual PC 2007
Virtual PC lets you create separate virtual machines on your Windows desktop, each of which virtualizes the hardware of a complete physical computer. Use virtual machines to run operating systems such as MS-DOS, Windows, and OS/2. You can run multiple operating systems at once on a single physical computer and switch between them as easily as switching applications instantly, with a mouse click.
I always use this to test new software. I have a clean XP installation in a safe place and use a copy of that as my crashbox. Every once in a while I throw away the crashbox and make a new copy of my clean installation. Works like a charm and keep my real installation much cleaner and tidier.