In Windows World there is such a thing as a VHD file (Virtual Hard Disk/Drive).  It is the backing store for a virtual hard disk drive that is accessible from one or the other Virtual Machines that Microsoft offers to include Hyper-V and Windows XP Mode with Virtual PC.  One might be tempted think that a VHD is a VHD is a VHD.  But no.  You usually cannot plug a VHD from one VM solution into another solution and expect it to work.  And, by golly, that's been verified.  Lying outside the comfort of the Microsoft arena are other VM solutions such as VMWare.  There are "solutions" that claim to convert a VHD to the VMWare equivalent.  They are not specific as to the restrictions and at least one has been shown to crash after starting to perform the conversion.  Some utilities exist that purport to be able to extract files (or entire directories) from a VHD.   In the best of all possible worlds there would be less confusion and actual standards that relate to VHD format.  Finally, the VM can be running, hibernating, or shut down so as to interfere with VHD conversion.  And the VM can have so-called "integration" turned on, turned off but installed, or not installed to similar effect.  (Integration has to do with the way the operating system on the bare metal interacts with the operating system running on the VM.  For example, you can cut a phrase on the real machine and paste it on the VM.)  My latest essay into the virtual world had me pursuing a path from Windows XP Mode with Virtual PC to VMWare.  Is conceptually simple and practically impossible.