Why Does Screen Sometimes Dim / Gray / Translucent ?

I just installed windows 7 64bit and have been further installing anti-malware and essential programs and drivers but have not finished (I still have unrecognized devices and am mostly using whatever drivers that Windows found on its own.

Every once in a while the entire screen will seem to dim as if you were looking through a cloud. I suspect it is some Aero effect, but I would like to know why it does that as it can happen when I am not using eithre the mouse or the keyboard. It seemingly just does it on its own. It only lasts a second or two.