“Windows 2000 Server and Advanced Server virtual machines display an Operating System property of 'Unknown.' If the Operating System property is set manually, it’ll revert to 'Unknown' following an automatic SCVMM refresh. There’re no errors associated with this change. Cause: 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,” informs Microsoft.