VMWare, VirtualPC, Virtual Server, Parallels - they all have one thing in common: in order to work faster, they install a set of drivers that patch the VM. This has a very positive effect on the virtual experience, but what happens when, say - the VMWare drivers are running under Virtual Server?
As you might have guessed it, no good things happen.
Products such as Parallels and VMWare Virtual Server allow you to import machines that were created with Microsoft's virtualization products. If you grab a machine from Microsoft's VHD test drive program and import it right away, you are in for a nasty surprise. The drivers for the previous product are still there, and they *will* conflict when you install the additions of the new product.
So remember, get rid of your product's "additions" *before* migrating virtual machines.