Restart in safe mode (upon computer power up - keep tapping the F8 key and select safe mode from listing with arrow keys - hit enter 2x) and then run defrag.
Warning:
If you have a large desktop (over 800x600) with icons all over the place, you may have to re-arrange them upon rebooting in normal mode - minor inconvenience.
If you are looking for superfluous files, try x:documents and settings\your logon name\local settings\temp and x:windows\temp and delete all you find here - where X is the letter of the Windows drive and 'your logon name' is what appears at the top of the Start Menu.
Good Luck !