Disk defragmenter is designed to run without you knowing about it. It will only run when you aren't using the computer so how do you know that it isn't? Are you experiencing performance issues? If you are really convinced that it isn't running try this: Start Menu > Accessories > System Tools > Task scheduler
In task scheduler on left hand side: Task scheduler library > Microsoft > Windows > Defrag
In the top center section look for ScheduledDefrag Rick click on it and select enabled. Double click on it, select triggers tab, and then the edit button. Make sure that enabled is checked. Close the windows.