Dispose how? If you are going to junk it, then remove the drive and whack it with a hammer a few times.
If you are going to give it to someone (not many people would want an old XP computer) then formatting the drive when you reinstall XP should be sufficient if you do a log format and not a quick format.
If you are OC about these things you can run a program that does a multi-pass wipe of the drive before you reinstall XP.