Windows Virtual PC 7 is a feature of Windows 7 Pro, Enterprise and Ultimate designed to support the XP Mode feature, as well as running older or other versions of Windows within your Windows 7 environment.

Probably the best known feature coming is XP mode, where you get a fully integrated XP  virtual machine to run software with compatibility issues in Windows 7. The big feature is the seamless integration, you won’t even know that the program is actually running in Virtual XP. (Kind of like Unity mode in VMWare)

You can get the beta if you are running the Windows 7 RC here, once you click the big orange button there, you will get links to install Virtual PC, as well as XP Mode.

Just a note, ensure you read the BIOS setup information and everything else there, this needs Hardware Virtualization support on the CPU to work, the articles will tell you how to find out if you have that.