I know a while back I had an issue where for no apparent reason if I played any form of divx file my screen resolution would 'fall off' my monitor - by which I mean that the entire 1600x1200 screen image would drop down the screen, leaving a full half of the top of the screen black, and roughly a quater of the left hand edge black as well.....and all id get is the corner of my desktop in the lower right corner.....
Changed codecs, drivers the works.....only reinstall fixed it!