>I have a strange thing on my computer (not the first time though). I see a white rectangle covering the upper left corner of the desktop. The only way I can get rid of it is by either rebooting computer or when I click on the "Show Desktop" on the system tray. I checked in the Task Manager if any application is running and see nothing unusual. If press on Alt-Tab to scan though applications, one of them will be this "white spot on the desktop".
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>What could it be?

I've seen both google chrome and internet explorer do that along with the windows explorer - seems like when it happened to me it was going from using 2 monitors to 1 monitor (or the opposite) ...Next time it happens go to your task-manger again and under processes tab start killing things to see which one it is. If right click on the desktop and click refresh does it go away then? My guess is it does not since you can alt+tab to it....