Sounds like you have copied the differencing disk for a virtual hard disk but not the associated base image.
For example, Windows XP Mode (on Windows 7) by default creates a differencing disk when you create a new VM based on the Windows XP Mode VHD that you download. See: http://technet.microsoft.com/en-us/library/ee449422(WS.10).aspx
IF you have Windows 7, and it was a Windows XP Mode VHD you have now, you might try downloading Windows XP Mode from MS and registering the base VHD, which might do something useful, otherwise you are stuck with getting more files from the school computers.
You will have activation problems running the Virtual PC VHD anyway, so probably best to run it with Virtual PC directly.