When you right-click on a drive in (My) Computer, choose Properties, click Tools and click Defragment Now…, the Defragmenter GUI is supposed to open. If the Task Scheduler service is disabled, clicking the Defragment Now… button may show the User Account Control dialog, but the Disk Defragmenter GUI does not launch. Result will be same if you run the file dfrgui.exe manually.

Fix: Defragmenter window does not open
If the Disk Defragmenter window does not open in your Windows Vista PC, check the status of the Task Scheduler service. Use these steps:
1. Click Start, type Services.msc and press ENTER
2. Double-click Task Scheduler
3. The Startup type of the service should be set to Automatic
If the service is set to Disabled, and the Startup type dropdown options are grayed out, use the REG fix (scheduler.reg) available in article Task Scheduler service properties are grayed out in Windows Vista. Be sure to restart Windows after applying the registry fix. Repeat the steps 1-3 to check the status of the Task Scheduler service.
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You must check the dependencies for the task that you are trying to enable… the task cannot run if its dependencies arent running. Go enable the dependencies and start them, then go back and start the task you wanted.
This should work.
when im at the services thing and then i double clicked on the task manager, and pressed start it then said “Windows could not start the task scheduler service on lan computer Error 1068 the dependency service or group failed to start” so please help me :(