Nicoliani
The process of converting 220/110 volts to the 12/5/3.3 volts required for
the computer internals generates heat which then has to be dissipated.. this
task is done by the power supply fan..
CPUs, hard drives and video cards also generate heat, and the power supply
fan helps to draw it out via the power supply housing..
From the above, you must surely be able to work out that turning off the
power supply fan while the computer is powered up would be the equivalent of
electronic suicide..
If you are looking to save on electricity usage, consider low energy light
bulbs, but do not use them where they will be subject to constant on-off
operation.. they do not reach full brightness immediately, and they require
more initial power to start up..