Mouse cursor keeps drifting

My mouse cursor keeps jumping all over the place. It’ll be at the top of the screen and then suddenly snap to the bottom. There’s no pattern to when or where it happens, but it’s getting pretty annoying. This happened back on Windows XP and it’s still happening now on Windows 7, even after I wiped my C drive and did a fresh install. Could this be a virus or something else entirely? I'm running some free antivirus software right now.

How old is the mouse?
When was it last cleaned?

Optical mice can sometimes act up—like the cursor jumping all over the screen—if there's any debris blocking the sensor.