All computers require a power supply of some kind, be it mains supply or battery. In most desktops there will be a steel box which take in the mains supply 90 to 250vAC and converts it to the internal low DC voltages required by the computer components. As this Power Supply Unit or PSU has to do a lot of work, it gets hot and therefore requires cooling.

The more power the computer draws the hotter it gets and the more cooling it requires.

In order to achieve this cooling, the fan(s) built into the PSU need to suck in cool air and blow out the hot air. One of the most common causes of PC failure is overheating due to lack of cooling or indeed lack of airflow through the PC.

If your PC's fans seem to be running a lot more now than when the PC was new, it could be dust has clogged up the inlet or exhaust ventilation holes. The inlet is usually at the front and the exhaust the rear. Sometimes a PC is crammed into a small place under a desk or indeed even in a cupboard, this will restrict the airflow and make the PSU overheat. This could just kill the PSU when the components cannot be cooled enough and they fail but that same failure can cause damage to the motherboard or disk drives.

A very clogged up air vent on a PC

So regularly clear the vents with a vacuum cleaner and keep the rear of the PC clear of too many cables and other debris.

Laptops too require their air vents to be kept clear of fluff and dirt and should never be used directly on a "Lap" as clothing can restrict the ventilation. Use it on a tray or a board where air can flow underneath and out the rear or the side.

Power supplies in desktops tend to run all the time, even when you shut the PC down. Doing START > SHUTDOWN on a Windows PC merely puts the power supply into a very low power mode. The only way to completely shut it off is to switch it off at the socket or pull the plug. For this reason sometimes power supplies fail when switched on after being off for a long period. So unless you really need save the few pence a year they cost to leave switched on in low power 'Off' mode, don't pull the plug.

When a PSU fails, most often the PC just won't switch on. As mentioned before, they can fail if they overheat due to blocked vents or a failed fan. They're not difficult to replace just awkward to get at and disentangle the wiring loom. If you ever upgrady your PC and choose to dispose of the old one, take the PSU out first, it might come in handy, even just for testing purposes.