If your running Win 98 .. here's my two cents

Probably because something in the background is running that is accessing the drive, causing the drive contents to change, and defrag to keep starting over. Realplayer and realjukebox are famous for this.
Try doing your defrags with nothing but explorer and systray running.... and kill all screensavers too.
1. Go to System Configuration Utility/StartUp and see how many items you have checked off ... unchecked as many as you can, [Your not deleting anything and can always be rechecked]
2.Make sure screen savers are turn off
3. Do a scan disk ...
4. Do a DeFrag ... if you can't ... do a search and find a file called "applog" and rename it "applogOLD" or something like that and do a defrag again ...this time their should be no problems.
http://support.microsoft.com/default.aspx?scid=kb;EN-US;q2181605. Reboot while hold down the "Ctrl Key" .. and when you get in to the DOS window ... choose item #5 and at the C Prompt type
scanreg /fix ... when the program is finished type in "win" at the C prompt and it will get you back to windows.
Should be no problems at this point.
sprint
*MOL*
PS: Let us know what happen