What is the difference between "Windows Virtual PC" (RTM released today) and "Microsoft Virtual PC"?
Windows Virtual PC is the new release of Virtual PC, only available for Windows 7.
Its main new feature is "Windows XP mode", which creates a tight integration between a Windows 7 host and a Windows XP guest, allowing the user to seamlessly run applications inside the XP VM directly from the host system, without having to manually create, configure, turn on and access the whole VM.
Windows Virtual PC requires a Windows 7 system and a CPU which supports Intel VT or AMD-V.
You can only use one of them. Before you use one of them, make sure the other one is uninstalled, or else you will end up with compatibility problems.