It is essential with modern computers that the hot air produced from the components is circulated so that it does not build up and result in overheating and burning out or causing it to lead to early failure by being overstressed.
For example the die on top of the processor gives out a great number of watts in a very concentrated area. To cool this you always need a thin layer of thermal paste and a heat sink and rated fan to draw the hot air away from the processor and other system components.
To cool something down you either need to apply a fan to circulate air or in a car engine it is cooled by water pipes with coolant being pumped around the engine, a cooling fan at the front drawing cooler air in to cool the hot air from around the engine.
Using Air to cool a PC is the most popular method as it is a cheap and reliable method.
Along with the CPU coolers case fans are used extensively inside the PC housing, to ensure sufficient airflow through out the case often one fan is placed at the front of the case to draw air inside with another fan at the rear or higher up set to turn to other way to move warm air to the outside of the case.
Additional fans are often used near the hard drives as these also run very hot now with increasing speeds. You may also notice that on your motherboard you have a number of chips namely the chipset chips, as with all devices they are using power to work and a consequence is the warm air which is given off some vendors are including smaller fans which are fixed on-top of the chipsets or bridges to ensure they are cooled sufficiently.
Graphic cards are now often larger with built-in memory modules and running more functions quicker to keep this in working order many cards now come with a cooling fan fitted to keep the chips cool and operating.
Cooling a PC can work in a very similar way, the main purpose is to ensure that for the processor the compulsory heat sink and fan is rated correctly. This means that it can produce enough airflow onto the heat sink to cool the processor. The circulating airflow moves the warm air away from the processor and is often trapped within the PC Case.
Depending on how many devices you have to cool you can either install a small number of powerful fans that are likely to be louder or several weaker fans that are much quieter. Fans come in various sizes to fit all situations, air cooling is all about passing air over the component which will move the warm air away and into the case.
3-4 Fan Standard Set-ups
The standard for most computers now in addition to the CPU fan is it will have one or two exhaust fans which will force warm air from the inside of the case to the outside. Often one or two fans are now built into the Power Supply. (PSU) Not only does this keep the PSU cool but it draws air from the inside through the PSU and out acting as an exhaust fan.
Also an additional fan is mounted at the front of the case to bring cooler air in so that it can be circulated and mix with the warmer air before being drawn out via the exhaust fans.
With 3 or 4 fans installed your system should remain very quiet, the CPU fan you can not do much about and you should just lave it to keep one of the main parts cool and working. For exhaust fans to either bring in or expel air you could either go for the standard options or look at variable speed fans which allows you to lower speed, as a result the noise will reduce and be quieter.
Prices for fans start from just a couple of £ and can increase to the £40-50 range depending on the size and specification.
For the more advanced or overclocker user who is comfortable with installing a kit from scratch or knows how it works to avoid any problems. You are basically installing a water pump / radiator style system into a case in a similar way that a car uses. Tubes, water and pumps either need to be installed in or next to your system.
Why cool a PC with water instead or normal air cooling? The main reason is that water cooling provides better cooling, the next is that by water cooling the system will run far quieter than with normal fans.
With these systems you can control the cooling system so much more and as a result push your system further by overclocking components further to squeeze every bit of speed increase out of the system.
Water cooling parts:
CPU Block - This transfers the heat from the CPU, essentially it is a block often copper with water running though the centre. The block has a hollowed interior where the water runs through and typically there are a number of barbs on the cooler which are used to connect the tubes to.
North Bridge Block - This part is often an optional addition and is normally used to replace the fan in an air based system. The chip itself does not run too hot unless you are looking to over clock.
Video Card Block - Again optional is designed to replace the cooler on your graphics card. Similar to the CPU block, this works in the same way be being secured onto you graphics card.
Pump - This pumps the water around your system. Things to look for are the flow, pressure, heat issues, and noise. All will be rated and the pressure and flow are vital depending on the amount of blocks and distance you need the water to pump through your case.
Radiator - Just like a car radiator this removes the heat from the water by separating the water into smaller parts where air can cool the pipes and as a result the water.
Fans and Shrouds - Shrouds are used to mount fans to the radiator, with fans being set an inch or two away from the radiator set to draw air away from the radiator.
Reservoir - This is the storage area for excess water; these are not essential but are normally preferred as with one there is less upkeep. Again like a car this should be a sealed system with no air.
Fluid - You can't just use any fluid or water. You should use distilled water or de-ionized water. The reason is these are less conductive than drinking/boiled water, so if water was spilled or leaked the chances of saving the rest of your computer are far higher with distilled water or de-ionized water.
Clamps - Not essential but they help keep the various tubes in place.
Tubing - Connect the different blocks and parts together and carries the fluid.
As you can tell from this short summary you will need to source a large number of parts to put together a watercooled system for your PC. And the size of the kits can vary vastly in price from £70+ for a system for an average tower system.