The Administrator's answer is exactly right....run defrag in safe mode. I would suggest one other thing, too. Do a good disk cleanup first (empty Temp, Temp Internet Files, Recycle, and whatever else you don't need. Also uninstall any programs you don't use any more (Start>Control Panel>Add/Remove Programs, then highlight the unused program, then click remove). I even go so far as to empty the Recent folder (My Computer>Maindisk c:>Windows>Recent). The more you get rid of, the faster it will be when you finally do run defrag.
Anyway....just a thought. Best....Larry