What operating system are you using?
If its windows 98, bootup your pc and keep pressing the F8 button this will take you into 'safe mode'. Try defraging from there. To get out of 'safe mode just restart your PC
If you want to defrag from Windows, once booted up, hold down the ctrl+alt+del button and clear everything in the box that appears except systray and explorer. (you have to used crtl+alt+del everytime you need the box to appear)
Try not to press ctrl+alt+del more than once at a time otherwise you will restart your pc.
Then run defrag.