I use full screen with my game (1440x900 - W7 with Aero active), and every time I need to ALT+TAB to check the Wiki and such, I return back and plenty of graphical glitches appear, from the avatars of my party friends being replaced by random scrambled graphics, to the item slots being missplaced or to this:
http://img33.imageshack.us/img33/5538/spiral20111011011942.png
It doesn't affect my gameplay too much, but when the item slots are scrambled, I can't remember where a particular vial was placed before I launch a Shock one against the Quicksilvers, so it turns kinda dangerous.
What video card do you have? But if I had to wager a guess, its probably the drivers if its only happening when you drop out of full screen mode.