I'm on Windows 7 64-bit and sometimes I experience about two second freeze when UAC dialog pops up. The computer stutters for about two seconds before showing the dialog and another two seconds after closing the dialog. The sound starts looping and mouse pointer can't be moved. This can be fixed with a reboot, but it's quite annoying since it happens every few days. When the problem appears, it'll be with all the UAC dialogs until you reboot the computer.
I'm running MSE and Outpost Firewall with defense mechanisms disabled. I do reboot my computer every night.