Once is a while something will alter your screen resolution. I don't know if this is what happened to you, but try this and see.
Click on Start, Control Panel, and Display. In display, click on the tab marked "settings". In the center of that tab will be your screen resoluation. Today most monitors run best at 1280 x 1024. If you monitor is a little older, you may want 800 x 600. If the numbers are lower than that, then that might be your problem.
If you graphic card operates on a high resolution, but the settings are too low, you would only see part of what you are accustomed to seeing.
If that doesn't work, go to trendmicro.com. Perform a free virus check. That virus check may pick up something your anit-virus program is not finding.