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:
Support for x64 Windows as a host operating system
Support for hardware virtualization support
Support for Windows Vista as a guest and host operating system
Support for PXE network booting of virtual machines
Support for the use of full screen virtual machines on multi-monitor systems
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.