By default on Windows 7, Scheduled task for Defragmenting local HDDs is enabled to run Every Wednesday at 01:00 AM.
You can disable this task using following command:
schtasks /change /tn "microsoft\windows\defrag\ScheduledDefrag" /disable
If you want to disable ScheduledDefrag to the Domain computers in your company then create a BAT file and include it as computer based GPO logon script.
After applying this GPO to the target computer run taskschd.msc. The status of ScheduledDefrag is disabled.