Are you running defrag in safe mode with the screen saver and power conserv. off. Alot of times if you have something going in the background this can make defrag stop. Also, your os or file structure could be damaged from when you had a virus, sometimes even after you remove a virus there can be problems, think about when you take cancer out of a person. You may want to try a safe recovery, this will let you repair any damged system files with out having to restore everything.