Were you trying to do anything that would have taken up the full screen at the time, like playing a game or watching a movie full screen?

A reboot would most likely fix it, but if it is indeed a virus, you'll want to update your virus definitions to the latest re-run a scan.
It may also have something to do with your graphics drivers, so you might need to update them. Hopefully your graphics card isn't dying.

In my best opinion, it would have been a software glitch that will probably seem to fix itself soon, if not already.