I reinstalled windows xp earlier today, and at some point in the past couple hours I notice a certain area of the screen about a square inch will suddenly get stuck displaying that portion of the screen from another window. it'll then change to that square image whenever I go to another window or to the desktop. it'll go away for a second while I move the scrollbar, but when I stop it'll show up again. during games, it'll flicker with every frame. I've tried reinstalling the nvidia drivers. what is the name for this phenomenon? what could cause it?