Abhilfe schafft hier:
"I had the same problem and suspected it was related to the fact that Virtual PC doesn't support Multi-CPU guest OS and the Vista image that I created was from a Core2Duo system.
I mounted (attached) the VHD using Windows 7 diskpart utility then renamed the active hal.dll file in c:\windows\system32 to hal-multi.dll (it should be safe to just delete it since it's really just a copy of halmacpi.dll that is in the same directory but I was being extra cautious). I then made a copy of halacpi.dll (the single-CPU hal.dll) and renamed it hal.dll. I then unmounted the vdisk and attempted to boot the VM again. It worked! :)"