When I've tried to run disk defragmenter recently, it will always stall saying "drives contents changed, restarting"! It must have been running for almost 4 -5 hours the other day and never completed - it's driving me potty! I've disabled MSN messenger and zone alarm in case they're causing problems but nothing seems to make a difference. The pc is a Compaq Presario model (about 3-4 years old now)with a 5gb hard drive.
try control alt and delete and then try closing all programs except - system, explorer and systray i think thats all you need to then run defrag with no problems once its completed just restart your computer to be able to use windows again