Black Screen when logging off but if..............

> Hello,
> I am running the TechNet Plus version (RTM) of Vista and it's working
> great except if I click to log off the computer says it's logging off and
> then the screen goes Black.....I don't think it's locking up but I can't
> really tell. I have to re-boot to get back to the login screen.
> If I use the windows key + L it logs right off to the login screen with
> options, as it should.
> Anyone have this problem or know of fix?
> Thanks,
> Craig