A fact often overlooked is that over time, computers accumulate large amounts of dust within the case. This dust blocks air vents, clogs fans and heatsinks, and thus leads to to overheating of components. In many instances this overheating will eventually lead-to erratic behaviour, becoming progressively worse until regular freezing or bluescreening start. Such symptoms are often taken to indicate a software bug or virus, leading to a merry chase around-the-block for an obscure software problem, when the fault is actually a straightforward, physical one.
The usual approach to desktop-PC dust removal is that of vacuuming. Unfortunately the typical domestic or industrial vacuum-cleaner gives insufficient suction to do a good job of removing the dust. There then arises the need to use a soft brush to disturb the dust so that it can be lifted. While the vacuum-and-paintbrush approach does work, it is not all that effective, and not entirely free of the risk of damage. Firstly, some dismantling is almost always needed, as a brush will not properly reach the processor heatsink via the fan. Key issue to watch here is risk of pulling jumpers off the mainboard, causing faults or misconfiguration. There is also the concern that brushing may drive some of the dust deeper into the works, from where it will be very difficult to extract. All in all, a time-consuming approach which is better than nothing, but doesn't do all that good a job.
A better approach, where facilities exist, is to clean by air-blast. This has to be done outdoors and clear of any contaminatable articles. The volume of dust created simply could not be tolerated inside a room. Air-blast cleaning can be done quite effectively with the 'blow' outlet of a vacuum cleaner. If using this method, remember that the first use of the cleaner in blow mode may expel rubbish from the hose, so point it away from the computer -and people- until this has cleared. A key advantage of this method is that almost no dismantling is needed, beyond taking-off the side cover.
Best results of all are achieved with a compressed-air line of perhaps 30-50psi and blow-gun. A typical tyre-inflation line is ideal. Here, the nozzle should be kept around three to six feet away from the computer. Because of the high pressure, do NOT place the nozzle directly against any component, or damage may occur. In any event you won't have to, this method is so effective that the entire computer can be cleared of dust in a matter of less than a minute's work.
When using either air-blast method, ensure you are standing upwind of the computer, as the sheer volume of dust expelled can come as something of a surprise. It might also be a good idea to wear eye-protection. Probably best not to wear your job-interview suit, either.
Pay particular attention to the processor heatsink as this is where the dust-clogging is most severe. Also, ensure that the power-supply entry and exit grilles are cleared of dust.
A laptop can suffer the same kind of overheating problems, and typical additional symptoms here will be a fan which constantly runs at high speed, but with little or no airflow detectable at the fan's side or rear exhaust-grille.
Laptops typically use a cooling-system which relies on a heat-pipe to convey heat from the processor to a finned heatsink at the side or rear of the case. Here, a centrifugal fan blows air over the fins. The same fan also serves as an extractor, drawing heated air away from other components such as the video-processor chip. Thus if the fan is obstructed for any reason, all internal components will become very hot.
Owing to the need for specialist tools and a delicate approach, the cleaning of laptop fans is probably best left to a professional. Here we tend to use more gentle cleaning tools, such as an aerosol air-duster, or a moderate blow output from a vacuum. Airlines are probably not advised. A key issue with the centrifugal fans used in laptops is that although the fan itself may look relatively free of dust, there may be a solid plug of 'felt' -made up of compacted dust and carpet fibres- hiding inside the fan cowl. You won't blow this out, it's far too solid to shift with the likes of an air-duster can. In fact, it's not a good idea to try, as if you do succeed in dislodging the wad of felt it may jam the fan blades, making matters worse. The fan housing needs to be opened, or removed from the heatpipe in order to clean it internally.
If you do feel like tackling this yourself it's worth noting that you should avoid disturbing the heatpipe if at all possible. In most designs the fan housing can be separated without doing so. If the heatpipe is disturbed, the processor die and cooling plate need to be cleaned of old, hardened thermal compound, and fresh compound applied.
This 'plug' of dust in the fan outlet tends to build-up quite rapidly in laptops used on carpets or soft furnishings, but possibly less so when used on a hard surface. Owing to the confined space in a typical laptop case an obstructed fan will lead to severe overheating, and this may cause permanent hardware damage. Effects can, for example, include a processor that crashes regularly, shortening of the battery life, or a damaged DVD drive. Instances have even been seen where the case has started to melt and distort with the internal heat. Therefore it's not a bad idea to have your laptop inspected and, if found necessary, cleaned at least once a year. That said, in most cases you can tell when a problem is developing, as there will be a racing fan but little airflow from the fan grille when the computer is working hard.
Always disconnect the mains lead before starting, and on laptops, remove the battery. (The computer's power button does NOT in fact switch the PSU off) Never poke any metal object into a PSU, as its capacitors may remain charged for some time after disconnection. This in fact is another advantage of the air-blast method, it generally clears the entire PSU of dust without the need to take risks such as poking a brush into the PSU fan.
Try to avoid hitting the optical drives or hard-disks with a hard air-blast as these contain fragile mechanical components. If the dust is stubborn you can be more aggressive in using the airline on the PSU slots or processor heatsink, as these are not mechanically fragile. Some reports suggest that it's not a good idea to cause fans to run-up to screaming RPM by playing high-pressure air on them, though I've yet to see one fail for this reason. If you want to be ultra-careful you could lock them stationary with some rolled-up paper towel. Just don't foget to take it out afterwards!