Normally, the fan speed is controlled from the "Task Manager". Instead, there may be a fan control program installed. For example, then link below:

http://www.diefer.de/i8kfan/manual20/

There sometimes is a CPU Fan control icon in the Control panel depending on your computer brand/model.

Usually, it's controlled from within the BIOS which turns the fan on when there's heat build-up or working harder, etc. So be cautious...Slowing down the fan or reducing it's speed could potentially cause damage to the computer inards.

Hope this helps and let us know more..

Grif