Log Off & Switch user stopped working

Have Vista 64 bit Ultimate and recently when I try to switch user or log off
a user account the PC keeps running but the screen simply goes black and
hangs. Only way I can switch users is to hold down the power button to
power off the PC.

I'm assuming something has corrupted the registry? Any help would be
appreciated as it's a pain in the neck having to power off to switch users.

thanks

Steve