fan speed

I've got a home made rig here that is excessively noisy. The sound that the tower produces is so loud that if I want to read in the room I have to put ear plugs in. Oddly enough, the sound appears to be coming from a fan that was advertised as a specialty, near silent unit. I would like to slow down the fan to the point at which it becomes bearable, but I don't wish to put my system in jeopardy of over heating.

I checked out the bios and found these specs
42 Celsius- System Temp
43 Celsius- CPU Temp
Fan Speed- 4590 RPM
CPU fan speed- 0 RPM
Northbridge Fan- 8766 RPM

I suspect that the CPU fan sensor is either not working properly or is nonexistent, so I'm not too worried about that. The other two fan speeds seem excessively high though.

How should I go about quieting down my system without burning it up?

Most probably it is the northbridge fan who is causing the noise. The faster a fan runs, the more noise it makes. However, it would be unwise to lower your northbridge's fan speed as it is probably that high for a reason. Perhaps you could replace it. You can probably find a good northbridge fan for a reasonnable price.

There's no need to have a northbridge fan in almost all cases. Find a good sized heatsink for it instead.

Do you have a case fan? Consider getting rid of it, you probably don't need it, they're just for fags who obsess over low temperatures and\or overclock.

Does your PSU have anything other than a single 120mm fan? Consider getting a different one if so.

Finally, is your graphics card one of those screaming monsters with a tiny little 40mm fan? Look around for replacement heatsinks, I had a Radeon 9700 a few years back which got damn hot, but for $30 I bought a massive heatsink for the thing that allowed it to run fanless.

These should get rid of the biggest noise producers. After that you have hard drives, the CPU fan and the quality of the fans in the system to worry about, but I doubt you're that concerned about sound that you want to make the PC "silent".