1) Remove dust with pressurized air.
2) find out which fan makes the most noise by (CAREFULLY!) stopping them in turn with your finger(s).
3) replace fan with low-noise alternative.
4) repeat step 2 and 3 until you replaced all fans (CPU, PSU, videocard, case fans).
5) notice the noise your harddrive makes, and try to reduce that somehow.
6) Put in some dynomat or something similar to remove the last bit of sound.
Try <a href="http://www.silentpcreview.com">http://www.silentpcreview.com</a> for some silent alternatives. Generally speaking, Zalman CPU coolers (if possible for your CPU!), some kind of silent PSU, a silent alternative for your videocard cooler (zalman or arctic), and silent casefans (zalman, papst, panaflow) might be in order.
I have quite a silent PC myself (after loads of modifications), so if you need some specific pointers, just ask. I'd need some info on your system, though. What processor (socket!) do you have, what videocard cooling (passive/loud fan?), what PSU type (8cm fan at the back and light as a feather, or 12cm fan and heavy from all the cooling area?), how many case fans do you have, do you consider your system to be hot and *is* it hot? That sort of info.
