Hyper-V Program Manager
There is a known problem that can occur when trying to shutdown a physical computer running Virtual Server with a large number of virtual machines running. The issue is caused by default Virtual Server will attempt to 'Save state' all of the running virtual machines before closing - and with a large number of virtual machines running this can end up taking a long time. By default - Windows will kill a service that fails to shutdown gracefully within 20 seconds - and when it does this to Virtual Server the result is that your virtual machines behave like they had a power failure (with risk of data loss, etc).
To change this behavior you will need to use 'regedit' on the host computer and edit the [HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\WaitToKillService] entry. This registry entry defines the ammount of time - in miliseconds - to wait before killing a service.