Disk Cleanup Scheduled Task hangs

I have a user with Windows 2000 pro. They have disk cleanup as a scheduled task, occuring on startup. The gray dialog box appears asking which drive to cleanup, hit ok, and then it says calculating. Lots of disk activity, task manager shows it taking anywhere from 60-80% of the cpu, but the blue bar never moves, and it remains in this state until I reboot.

Other times, disk cleanup will execute just fine. Start to finish in just a few minutes.

Any thoughts?