1. Open Control Panel
2. Double-click Scheduled Tasks
3. Double-click Add Scheduled Task
4. On the Scheduled Task Wizard dialog, click Next
5. Click Browse
6. In the Select Program to Schedule dialog, navigate to the windows\system32 folder
7. Select defrag.exe
8. Click Open
9. In the Scheduled Task Wizard dialog, type a name for the scheduled task (Disk Defragmenter, for instance)
10. Under Perform this task, select how often you wish Disk Defragmenter to run
11. Click Next
12. Set the time at which you wish the Disk Defragmenter scheduled task
    to run. This should be a time when your computer is on, but not in
    heavy use.
13. Select the frequency at which you want the Disk Defragmenter
    scheduled task to run (Every Day, Weekdays, or Every days, where
    is the number of days between scheduled runs)
14. Click Next
15. Enter a user name under which the Disk Defragmenter scheduled task
    will run. Note: This user must be an administrator on the local
    machine.
16. Enter the password for the user you entered in the previous step
17. Confirm the password for the user
18. Click Next
19. Check Open advanced properties for this task when I click Finish
20. Click Finish
21. In the Run text box, you should see the full path and command for
    defrag.exe. By default, this path is C:\WINDOWS\SYSTEM32\defrag.exe
22. Append the drive letter for the drive you wish to defragment to the
    command in the Run text box. In a default installation, your Run
    command will look like this:
    C:\WINDOWS\SYSTEM32\defrag.exe C:
23. Click OK
24. In the Set Account Information dialog, enter and confirm the
    password for the user listed in Run as
25. Click OK

Source: http://support.microsoft.com/kb/555098