A couple of people I know have hit this problem - so I thought I would let you all know about it.  If you have Virtual PC / Virtual Server installed on a computer, and realize that you have not enabled hardware virtualization in the computers BIOS - make sure that you do not have any virtual machines that are in a saved state before you change the BIOS setting.

The problem is that when hardware virtualization is enabled, Virtual PC and Virtual Server use different structures internally to the saved state file, so saved states are no longer compatible if you change this BIOS setting.

Cheers,
Ben