1. Press the Windows button. Type 'regedit.exe' in the 'Start Search' field in Vista and Windows 7. Press 'Enter.'
2. In Windows XP, click the Start button, then select 'Run' and type 'regedit.exe' in the 'Open' field. Click on 'OK.'

3. Navigate to the following registry path on the left side of the window:HKEY_LOCAL_MACHINE >> SOFTWARE >> Microsoft >> Windows >> CurrentVersion >> explorer >> MyComputer >> DefragPath
4. Right-click on the registry entry 'default' on the right pane of the window.
5. Click on 'Modify.' Type the following in the 'Value Data' field:C:\Windows\System32\dfrgui.exe
6. Click 'OK.' Exit the Registry Editor. Restart your computer, and open the Disk Defragmenter to ensure that it is functional.