You can now dowload Microsoft Virtual PC 2004 Service Pack 1 software for free! Microsoft will also be offering Virtual PC 2007 for free.
For those unfamiliar with the technology, virtualization allows multiple operating systems at the same time on the same physical computer. This has numerous benefits, for example, it can be use to test software on multiple versions of an operating system prior to distribution, or to run a legacy application that only runs on an older operating system.