Feline,
First thing is to make sure you have enough room left on your hard drive. (Not having room on your hard drive will slow your computer way down.)
When you run Defrag, also disable your screensaver and antivirus program.
You can use the Run Command to run msconfig to see what processes are loading when you start.
Programs running in the background have been known to stop defrag from running.
Take Care,
Richard