Maximum PC

I was on my computer today and out of nowhere, a black square appeared in the top right corner of the screen, went away, then immediately came back and stayed there. All windows and the mouse go right under it. I haven't installed any new software lately (it seems like that's what has caused it for some people), and I don't think it's a problem with the monitor because when the computer is off, the entire screen turns that light black color, including the spot where the black square was. Here's my system, if it matters:

MSI H67A-G43 B3 Motherboard
Intel i5-2400 CPU (Quad Core)
GeForce GTX550 Ti

How can I fix this?

Edit: The black square is about half an inch away from the far right edge, and all the way up.