To leave the drive about 50% empty and try it in SAFE MODE.
I am having a problem getting Defrag to run on my C drive. I've tried it several times and it runs to completion but then when I check it again it is still very fragmented and says that I should defrag it. It reports that my drive is about 35% fragmented. I tried booting in Safe mode, thought that might do the trick but got same results, which means it did nothing. I am running Windows XP Pro SP2.