I've had this problem, too. Probably you have some program running in the background that intermittently accesses your hard disk, thereby interrupting Scandisk or Defrag and causing them to repeatedly restart.

The thing to do is restart your computer in Safe Mode, then run those programs, then restart in regular mode. Your Windows Help screens should explain how to do this. (I'd explain it myself, but you might be using a different version of Windows than the one I'm familiar with.)