After upgrading to SP2 and enabling protection features offered by NX extensions your copy of Virtual PC will not work anymore due to incompatibilities with the PAE mechanism and the NX extensions.
Developing smart devices applications with Visual Studio .NET using the emulator (that use Virtual PC) will be denied and so you must disable these extension to get it working again.

Open the C:\boot.ini with a text editor and change all lines like this

multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /fastdetect /NoExecute=OptIn

with the /NoExecute=AlwaysOff /NOPAE switch.

The result should appear like the sequent line:

multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /fastdetect /NoExecute=AlwaysOff /NOPAE

Reboot and have fun

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