What is Virtual PC?
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.

…and according Microsoft:
Whether Microsoft virtualization technology is an important component of your existing infrastructure or you’re just a Virtual PC enthusiast, you can now download Virtual PC 2004 absolutely free. Microsoft is also offering the free download of Virtual PC 2007, with support for Windows Vista in 2007.