Another way to solve this is to stop all programs except explorer - control/alt/delete brings up a box with all background programs, click on each to end task except for explorer. It works fine if you do this.
If you have any other progs running Windows defragmenter keeps restarting, and sometimes never finishes. I don't remember ever doing it in less than 40 minutes. Now it takes five or ten minutes at most, and I don't have to stop work to use it.