According to Paul Thurrott, a beta version of Virtual XP for Windows 7 has been released to MSDN and TechNet subscribers.
The download comes in two parts. The first is KB958559, an update for Windows 7 that includes Windows Virtual PC for Windows 7. The second, is another exe that installs the Windows XP with SP3 virtual machine itself.
To run Virtual XP Mode, users must be running Windows 7 Professional, Enterprise or Ultimate with preferably 2GB of RAM with at least 256MB available to virtualization, and a processor that supports hardware virtualization. Many times hardware virtualization needs to be enabled in the BIOS to be functional.
The bits for the RC was released to MSDN, TechNet and Connect memebrs today. A public beta of Virtual XP for Windows 7 is expected to be released to the public May 5, the same time Windows 7 RC will be released to the public.