It would help if you told us what friggin' OS you have...
How to Turn On or Off Vista Automatic Defragmentation
1. Click Start.
2. Click Control Panel.
4. Click System and Maintenance.
5. Under the Administrative Tools section at the bottom, click Defragment your hard drive.
6. Click OK to the UAC if it appears.
7. Click or unclick Run automatically (recommended) depending if you want this feature enabled or disabled.
8. Click OK
How to Turn On or Off Vista Automatic Defragmentation
1. Click Start.
2. Click Control Panel.
4. Click System and Maintenance.
5. Under the Administrative Tools section at the bottom, click Defragment your hard drive.
6. Click OK to the UAC if it appears.
7. Click or unclick Run automatically (recommended) depending if you want this feature enabled or disabled.
8. Click OK