When you have a video conversation open (I tested with the full video conversation option (The one with both cameras, voice and full screen option, but it might happen on other camera and/or voice modes)
And you start CCleaner it instantly throws Windows 7 in basic graphics mode (Disables Aero), It will restore Aero automaticlly after closing Messenger.
It might be a bug in CCleaner or it might be a bug in Messenger (Which I already saw trigger basic graphics mode when playing any of its conversation window games (FAIL)).
The message for basic graphics mode also dissappears quickly when this happens probabley related to the UAC prompt (Maybe its the UAC prompt triggering this?).
It can be CCleaner, Messenger, or the UAC prompt triggering it...
Using:
Windows 7 RTM
Messenger Live 2009 (Latest ATM)