Just a quick heads up on a new Knowledge Base article we published recently:
Windows 2000 Server and Advanced Server virtual machines display an Operating System property of 'Unknown.' If the Operating System property is set manually, it will revert to 'Unknown' following an automatic SCVMM refresh. There are no errors associated with this change.
A database entry that identifies the list of supported virtual machine operating systems per hypervisor requires an update.
For all the details and instructions on how to update the VirtualManagerDB see the following KB:
KB2025530 - System Center Virtual Machine Manager 2008 R2 identifies Windows 2000 virtual machine Operating System as Unknown
J.C. Hornbeck | System Center Knowledge Engineer