I have a laptop with a second monitor connected in extended mode. Every time a UAC prompt comes up, both monitors dim, then about 1 or 2 seconds after the prompt goes away, the extended configuration disappears, leaving me with only the laptop monitor on. I can re-enable the extended monitor manually, but this is a pain. Any ideas?
For future reference to a similar problem I had, my screen is "Samsung 24" LCD Syncmaster 2443BW", which is connected with VGA (Sue me) and I have to manually do some settings in the Catalyst control center in order to position the screen correctly, more specifically, under My VGA Displays->Image Adjustments and then click the arrow to the right to make the content of the screen align.
Why this is relevant is because when UAC pops up with the dimming enabled this setting for some reason resets. Here's how to disable the dimming as ackvictor figured out as a workaround: http://www.ehow.com/how_5517506_stop-dimming-screen-vista-windows.html