One of my customers did as well, I had to turn it back on, then back off and it started working again.
Basically the trick is to set it the opposite of what it is set to now and then back again. What ever this bug is from one of the Windows updates enabled the UAC but keeps it silent, never asking, so even when turned off it was still doing it.
Wont hurt to do it, just go to the UAC, put it back to the default level, hit apply and OK, then go right back in and turn it off, Windows will say you need to log off and back on, go ahead and reboot and see how it does.
I havent been able to replicate the bug here in vmware so I havent been able to try and find a fix other than what I said about flipping it on and off again.
Shane