scheduling defragment

Greetings,
I want to schedule a task which will defrag e.g.
say F: but I can not. Just I am able to open open disk defragmenter dialog at given time. But it doesn’t start defragmenting. ‘c:\winnt\system32\dfrg.msc f:’ by this command the f: driveis selected in the interface but can not activate the defragmenting. How to achieve this