Basic info for a quiet PC

Here's a component by component guide to having a quiet computer.

case
the case you choose is vital in determining how quiet your system will be. Good airflow through the case is a must as the quietness of your computer depends largely on how cool the components are. A good case for a quiet system would have 120mm fan mounts front and rear with unimpeded airflow through the case. A lot of cases have poorly designed hard drive bays which prevent air from freely flowing from the front to the rear of the case. A little modding usually solves this problem.
The material the case is made of also affects how noisy it is. An aluminium case will resonate easily, creating a very audible "hum". Steel cases or antec's aluminium/plastic composites resonate far less and are thus much quieter.

Acoustic sound absorbing foam can be used to quieten your case even more. The impact of acoustic foam on system noise is small but noticeable, especially on an aluminium case. The foam not only dissipates some of the sound from your internal components but also "deadens" the case panels and thus reduces case resonance. It can be quite heavy though, not really suitable for lanners.

list of some cases suitable for quiet pcs:
Antec P180
Antec P160
Antec SLK3000B
Silverstone TJ06
Coolermaster Stacker

case fans
Case fans are the primary sources of noise on the average loud pc. Replacing your case fans with good quality quiet fans will dramatically quieten your pc. Be warned though, quieter fans also push less air than louder ones, so you'll need to know roughly how hot your components are before deciding on which fans can adequately cool your system and thus which to buy.

The larger the fan, the quieter it can push the same amount of air as a smaller fan, so 120mm fans all round is the desired setup.

fangrills: this is a very important section. Most cases today unfortunately come with very restrictive stamped fan grills. These not only reduce airflow by about 20 percent but also create a LOT of wind cut noise. If your case has grills like these, cut them out with either a dremel or aviation snips. Place a default chromed fan grill over them, not any of those stupid laser cut skull grills etc. Anything placed in front of the air coming in or out of your fan is bad full stop, the only reason i'd even place the default unimpeding chromed grill over the fan when you're done cutting is for the safey of any curious cats.

mounting your fans: Hardmounting your fans to your case (i.e: just screwing them on) will transmit the vibrations from your fan directly to the case, which we want to avoid. Using soft rubber grommets will reduce noise. The fan can also be mounted using zip ties and blocks of foam, not a pretty solution but works more effectively than grommets.

fan speed adjustment: Fans run at 12v by default. This will make them spin faster than is required to cool your system. Slowing down your fans will reduce the noise they produce exponentially. This can be achieved by a variety of methods. You can use a drive bay fan controller which can control the speeds of around 6 or so fans. Zalman's fanmate 2 is another good option for individual fan control.
Some mainboards can adjust fan speeds themselves. Using a program called speedfan you can set up your system so that your fans speed up or slow down depending on your system temperatures, ensuring that your pc is always as quiet as possible and adequately cooled.

list of quiet fan makers:
Nexus
Globe
Panaflo
Papst



cpu and heatsink
First things first, pentium 4s are hot, athlon 64s cool. So if you've been paying attention so far you will automatically infer from this that an athlon64 system will be quieter than a pentium4. The athlon 64 winchester and venice cores are a silent pc enthusiast's dream, pentium 4 prescotts a nightmare.

Stock heatsinks and fans are generally inefficient and noisy. Replacing them with good heatpipe heatsinks and big quiet fans is a must. Athlon 64 venices and winchesters can often run passively cooled with a good heatsink and in a case with very good airflow. At worst they will need a barely audible slowed down fan to cool them. P4 prescotts will always need active fan cooling.

good heatsinks
Thermalright xp120
Thermalright xp90c
Scythe Ninja

psu
Most people know very little about what makes a good psu and their own requirements for a psu. A pentium 4 system with a high end graphics card and one or two hard drives will run fine on a 300W psu yet people rush out and get 550W psus for these systems thinking they need it.

This is a big mistake to make, especially if you want to have a
quiet pc. These high wattage pcs have very low efficiencies at low power draws (which is what your pc will be drawing 90% of the time). Low efficiency means more heat given out and thus a harder to cool pc and thus a louder pc. A lower wattage psu will be much more efficient in these ranges and make for a quieter system.

Efficiency is just one aspect, the noise produced by the psu itslef must also be considered. A lot of psus feature a dual 80mm fan system. These are louder than we want. psus with temperature controlled single 120mm fans are what we are after.

When it comes to efficient, quiet psus, one brand is easily the best, Seasonic. With 80%+ efficiency across the board and the quietest fans of similarly powered psus their new s12s are in a league of their own. Antec and enermax psus are also quite good and easier to find than the very rare seasonics.

note: Fanless psus such as those made by antec and silverstone are of course, quieter than the fan cooled seasonics. However you REALLY need to know your stuff before using one of these, your airflow around the psu must be very good. Don't buy one of these unless you either know what you're doing or are prepared to fry an expensive piece of hardware.

hard drives
If you've spent a good deal of time and money on silencing your pc, chances are that when you're done, your hard drive will be left as the noisiest component in your system.
Hard drive seeks are pretty noisy but fortunately there are a few ways you can quieten them a bit.

Decoupling your hard drive from your case is a MUST. A hardmounted hard drive is a massive source of system noise.
Decoupling your hard drive is the single most effective silencing mod you can do. You can do this by suspending your hard drive in a 5.25" drive bay using sewing elastic (don't use normal elastic bands, they dry and crumble over time) or taping your hard drive onto a soft foam base. This method will drastically reduce the vibrations being transmitted to the case and thus reduce case rattling and resonance.
For those in pursuit of silence at any cost, slowing down your hard drive speed is another option. Using Hitachi's aam drive tools you can slow down the drive a bit. This increases seek times marginally but reduces the noise produced by the drive itself and also the vibrations it transmits to the case.

Companies which make quiet hard drives are seagate and samsung.

northbridge
Most powerful motherboards sold these days have incredibly loud 40mm fans cooling the northbridge. These things can easily be the loudest component in any system.
Replacing this fan with a zalman chipset heatsink is an automatic mod for a quiet pc. This is usually sufficient to cool the northbridge however in a very high powered system it can fall short. If you use one of these, make sure to stress test your system to see if the heatsink is up to the task. If not, get a quiet fan mounted on the heatsink to get a bit of airflow around it.