Two things to try:
1. Run it after a reboot, as described in http://www.updatexp.com/windows-xp-chkdsk.html
Don't run it directly while being logged in.
2. Run it from your XP CD (recovery console) as described in http://kb.wisc.edu/helpdesk/page.php?id=5097
If it still gives the same error message, something might be wrong with your hard disk. You can try to mount it in external case or via an external cable and check it then. If that gives the same error, try formatting or delete the partition and create a new one, still having it as an external disk. If that fails also, it's time to buy a new one.
i get "windows was unable to complete disk check" when i try to scan disk drive 'c' but it works for "d" drive and the when i try to defrag i get a 'disk defrag could not start" for that drive also does any one know how i can fix that problem it has been like this for a week now