You can't. Operating systems don't have the ability to control the the RPM of a fan directly connected to the PSU or the mainboard.
You can install software that will <i>monitor</i> the fan speed, such as <A HREF="http://mbm.livewiredev.com/" target="_new">Motherboard Monitor</A>. But that's as far as it goes.
You'll need something like this to control the fans:
<A HREF="http://www.octools.com/index.cgi?caller=articles/baybus/baybus.htm" target="_new">6 Port Baybus Review</A>
Rotary switches are also nice, which is what I have with my system. This works well with the new <A HREF="http://www.mikhailtech.com/articles/other/enermaxfans/" target="_new">Enermax adjustable fans</A>.
Toejam31
<font color=red>First Rig:</font color=red> <A HREF="http://www.anandtech.com/mysystemrig.html?rigid=17935" target="_new"><font color=green>Toejam31's Devastating Dalek Destroyer</font color=green></A>
<font color=red>Second Rig:</font color=red> <A HREF="http://www.anandtech.com/mysystemrig.html?rigid=15942" target="_new"><font color=green>Toey's Dynamite DDR Duron</font color=green></A>
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<font color=purple>"Some push the envelope. Some just lick it. And some can't find the flap."</font color=purple>