Joyride,
Watch your scandisk and see if it is constantly going back to zero and restarting it's scan. If that is happening, then you have some process running in the background that is updating the disk, and resetting the dirty bit, which causes scandisk to restart from the beginning. Someone else mentioned running scandisk in safe mode. Or, if you are running Win9x/ME, do a C-A-D, and kill everything non-essential, then try running scandisk and/or speeddisk.