You can read more about Virtual PC 2007, and download it, from the newly updated Virtual PC website.
Some key features of Virtual PC 2007 include:
A long time ago Andrew Connell put together a great article HOWTO: Use Virtual PC's Differencing Disks to your Advantage. If you do anything with Virtual PC make sure you read this. Not only does it save you a great amount of disk space but it will save you a great deal of time when setting up new virtual images.