Windows 7 schedules the defrag to run when it assumes you won’t need the computer, usually around 3:00 a.m.
If you have your computer turned off at the appointed time, no defrag occurs.
Check the time that your defrag is scheduled to occur so that you can leave your PC turned on that night.
Defragging in Windows 7 is turned on by default.
You can make changes to suite your own schedule. Access the tool through the following steps:
(Choose Start→All Programs→Accessories→System Tools→Disk Defragmenter)