Here are some hints you should check:
a) Do you have enough free space left? Especially, do you have enough continous free space? See the cluster view for details: are the blocks scuttered around -> low continous free space, blocks are neatly arranged -> good free space conditions.
b) Are applications running while you defrag? Maybe they lock files accessed by the defragger?
c) Run scandisk to check your file system integrity. This should be okay, but just to be sure.
This was first published in June 2002