Microsoft is now offering Virtual PC 2004 Service Pack 1 for free. Virtual PC 2007, which supports Microsoft Vista, will be available in 2007 (also free).
With Virtual PC you can run multiple operating systems at the same time on the same physical computer.
These virtual machines can be used to run legacy applications, provide support, train users, and enhance quality assurance.
I guess Microsoft felt they needed to do this to retain market share after competitor VMwave released their free VMware Player in which you can run pre-built virtual applications and machines.