Create Scheduled Task:
-open Start\all programs\accesories\system tools\task scheduler
-double click Add Scheduled Task
-browse to (depending on XP version):
PRO -> C:\Windows\System32\dfrg.exe
HOME -> C:\Windows\System32\defrag.exe
-now set up how often you would like -once per month is probably enough-
-click next, then check the box to show advanced settings on finish
Set Drive for defrag:
*The program choice was simply to give you the correct options.
-change the comand line to defrag C: if you want to defrag the C:\ drive, or replace it with the desired drive letter.
-that's it, you won't get the pretty GUI MS designed, but instead a DOS prompt, but it will equate to the same end result... your drive is defragmented at a convenient time.
*If you wish to test your task, right click on it and press run, to stop the defrag press CTRL+C