Problem with Program Permissions in Vista

I'm running Vista in a Virtual PC 2007 window. I can run programs and
access files like expected, but if I leave the VM running for any
length of time, when I try to run a program (say Windows Media Player)
I get an error message saying I might not have sufficient privileges
to run that program. I don't get any subsequent dialogue window
regarding permission (like when installing a new program).

This has happened a couple of times. I've fixed the problem by
shutting down vista and rebooting the virtual machine window.

Thanks for any comments!
David
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