you don't have screensaver enabled do you?
sometimes windows screensaver when enabled, will make the windows OS disk defragmenter not start, keep stopping or starting over , will do the same for scandisk
if the Windows Screen Saver option is not enabled, then you might have a problem......
you might want to try getting a different disk defragmenter program too.... I myself use, the one that comes in my "Paragon HDD Manager™ 2009 Pro" software.......it goes in to a DEEP InDepth Defrag that defrags the system before the OS loads up to the desktop..... very thorough
my 2 cents.....