I found this and you may find it useful as well.
How to schedule disk defragmenter on Windows Server 2003
1. Open Notepad and add the following line
defrag C:
Save the file as defragC.cmd
2. Go into Scheduled Tasks and create a new Sched task (Add Schedule Task)
3. Browse to the location of the defragC.cmd and then modify the schedule to run it at whatever time you want
