Chdsk won't work/can't defrag

I have been unable to defrag my home computer (Windows XP service pack 2), which it seriously needs. i have tried both the Windows program and a third-party utility (Fix-It v. 9). each time i try, the computer indicates that chkdsk is scheduled to run and that i should first run chkdskf. i then try to run chkdsk from "start" "run" and it does not recognize that program. it allows me to run chkdsk in read only more, which works until about midway through the second step when it encounters errore and states it must shut down. i've tried a few variants on chkdsk, such as "chkdskCf", and nothing seems to enable chkdsk to run. i read somewhere that i should try from a DOS window. i think i figured out how to do that, but have been unable to get chkdsk to run.

any ideas?

thanks.

try a program called defraggler - http://www.defraggler.com/

thanks. i'll look into it.