A kit that contains everything you need for CPU cooling for as little as £60, but expect to pay almost as much again for a block to cool a high-end 3D card. If you're after total silence, these lower- end models won't do, as they still sport a slow fan attached to their radiator, but drop a couple of ton and you can pick up an external watercooling kit such as the Zalman Reserator.
This ferries the liquid out to a large, external, passively-cooled box. While this takes up a chunk of room and means you won't be carrying your PC to any LAN parties in a hurry, it will make your PC almost entirely noiseless. While you can go even crazier and adopt nitrogen cooling or a submerged system, that's more the realm of crazy overclocking than simple silencing.
Silence and quietening are two very different things. There's something a little obsessive-compulsive about total silencing, but it can get to even the most cavalier system-builder. Once you're rid of one problem, you get hung up on the next seemingly noisy component, and so on. Forcing yourself to think more practically, you'll find that reducing noise to a low background hum is actually very straightforward. All the better to drown out with the calming tones of zombie genocide or racecar engines.
In The Beginning
Is your PC being cooled too little or – hopefully – too much? The PC Health screen of the BIOS or a free app such as CPU-Cool will tell you – really you don't want the processor to be running higher than 50ºC even under heavy load. If it's idling around 30- 40, it may well be that you can simply unplug a case fan or two, or that you can reduce the CPU fan speed (either with software or a hardwired fan controller) without causing any problems.
If that's not an option, then replacing the case fans might knock out the worst of the noise. In pretty much every case, the fans are a standard size – either 80mm or 120mm – with a standard fitting. So find yourself a nice fan billed as super-quiet, remove the screw from each corner of the old one then bung the new one on instead: as simple as that. Next: if you're running a stock Intel or AMD processor fan, it needs replacing: simple as that. It's no good for overclocking and makes a noise like a dying snake.
A decent replacement doesn't cost much, but the majority are significantly larger, which is something to consider if there isn't much space inside your case. A few butt against graphics cards and RAM too, so make sure you can return it should it turn out that your motherboard layout doesn't take it.
No Moving Parts
Once upon a time, passive cooling was all it took. Those days aren't quite as long gone as you might think, but the teeny cooling blocks of yesteryear have been replaced with vast heatsinks the size of paperback books. You can viably replace quite a few of your system's fans with passive coolers – the motherboard's northbridge and the graphics card are the easiest to achieve.
If you're trying to go silent but don't need a high-end gaming PC, you can buy a cheap 3D card that's already passively cooled – a GeForce 9600GT will be about as good as it gets, which is enough for WoW or TF2. If you want to passively cool something meatier, you'll need to be prepared to spend a fair whack of cash to do so, and often for the card to take up an extra upgrade slot as a result.