PROBLEM

Everytime disk defragmenter is run it keeps getting so far through and then stops and restarts again from 0%.

CAUSE

Another program could be running in the background

FAT table may be corrupt, files may be cross-linked, drivespace is being misreported.

SOLUTION

Hit CTRL-ALT-DELETE and end task all programs except for Explorer and Systray and try running Defrag again.

If it still keeps stopping and restarting then run Scandisk with surface scan enabled. It might find errors like cross-linked files, corrupt FAT table, free drivespace being misreported. Once you've ran scan disk, try running Defrag again.