I have an install of Windows Server 2003 setup under Virtual Server 2005. I use this image for continuous integration of my Connector/Net products. The host machine is running Vista x32 Ultimate.
I'm not sure if its because it's on Vista but any virtual machines I add would disappear following a reboot. Not only that but attempting to re-add the machine would give an error indicating that the machines configuration was already present and could not be added again. Fun.
There's an easy solution though. Just move your machine images into the default search path for Virtual Server. On my machine that's C:\Users\Public\Documents\Shared Virtual Machines. If you've already worked around this by renaming your .vmc file a few times, you can clean out those old configurations by deleting appropriately from C:\ProgramData\Microsoft\Virtual Server\Virtual Machines.
[BTW, I'm using Virtual Server because VMWare Server doesn't support Vista. And, yes, I know that some have got it to work but it's not supported.]