Ive replaced one before, but it was dieing.. First, take the old fan out... Then buy a fan of equal size, if the power isnt the same pin size as the old, find an edge to the Power Supply that isnt welded, using a flathead screwdriver towards the top or wherever, open up just a small spot for the cable. Take the cable all the way down to another power source that will work.
I used it that way---bought a fan from plycon.com that had heatsensing, and it worked great..
if you cannot replace the fan or you cant get into it then probably the only thing is to replace the whole psu or you can cut some of the wires inside the psu to make it run at 7 volts instead of 12 this will make the fan run slower, troll the net and you will probably find out how to do it but i think it may be a bit dodgy to say the least anyway here is a site that you can try where you can get a silent psu 300w
hope this helps you a little bit