If this is XP home, in normal use, there is only one user (unless you added some)
You can;t really delete that user, as in normal operation, there would then be none. you would need to add a new user then delete the old one.
In XP pro and home, users should be deleted thru the computer management control panel applet. This will not remove their folders in documents and settings/ users however. these must be removed manually.