>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?

Try to update the video driver.