This morning while trying to spin up one of my Virutal PCs I ran into some peculiar error messages. In case the same thing happens to you, the workaround is pretty simple.
When I opened Virtual PC and selected my VMC file I received the following message:
Could not register the virtual machine. The virtual machine configuration could not be added. User does not have sufficient access rights.
Even if I ran Virtual PC as administrator I still got that error. The fix is to create a new virtual machine. When you do so, make sure to point to your existing VHD file. At this point you’ll probaly get this message:
Virtual PC . . . was unable to write to one of its virtual hard disks.
The fix here is to reference the VHD file by share name, not drive name. So instead of simply pointing to C:MyVirtualPCsMyFavoriteVirtualPC.vhd, you have to point to \ComputerName:MyVirtualPCsMyFavoriteVirtualPC.vhd. Now, since I keep my VHD files on an external hard drive, this meant that I had to create a share on its parent directory.
Thanks to Damir Dobric for posting the solution!