Are you using the windows default defrag program or have you got something else installed like diskeeper?
Cause if the windows built in defrag utility is gone, i think you may have a hard time getting it back without re-installing.
If its not there why not download a third part app like app like defraggler ; http://www.piriform.com/defraggler/update?v=1.15.163&l=1033
there is also ;http://www.google.sc/search?q=windows+defragmenter+missing&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:unofficial&client=firefox-a
and this link ;http://forums.techarena.in/windows-xp-support/906490.htm
which seems to have had similar problems.