A friend came over with some rebuild/salvage type computers that supposedly worked. One proved to be defective right off. (Read the second part of my hardware guide for hardware testing steps.) He said he blew out the systems with an air compressor before he brought them over and was engulfed in a dust cloud (I've had that happen many times before).
Popping the hood of the case and looking at the motherboard, it became apparent why the bad system was having problems. It had 5 leaking and damaged capacitors. While capacitor problems are common in cheap motherboards, I believe these capacitors had some help in their death. There were still some hair balls left in the case under some parts and in some cracks. These capacitors were probably over heated and cooked.
He mentioned the previous owners had a small dog. I looked at the whispy hairs. Most were 2-3" long. Some were white and some where light brown. I looked him in the eye and said "pomeranean". We both had a very hard laugh at that point.
While this may sound funny, the reality is this: Computers are better air filters than real air filters (read up on ionizers and charged deposition). That may be great for the humans, but it is bad for the hardware.
Dust gets into fans and acts like fine sand paper. This wears out bearings faster. The dust can also thicken up the bearing grease into a paste and make the fan work harder. Hair gets into fans and acts like a brake. Pets shed more than humans, but humans aren't entirely innocent here. In extreme cases this will be so bad that the fan won't even turn. When air flow is reduced, parts will over heat and system stability will be compromised.
Hair balls and dust bunnies (usually together) that build up around heat sinks can get so bad as to block air flow. Hair balls provide very good structure for dust plugs. Sometimes these have to be pulled out with tweezers.
Dust that covers parts will start acting like an insulator. This jacket effect will lead to over heating problems. Heat literally cooks parts and will greatly shorten their life. Over heating can also cause random instablility in the hardware.
Some types of dust can become conductive in sufficient quantities (especially in high humidity and wet climates). This can lead to signal quality problems or even shorted out wires/pins in extreme cases.
Second hand smoke is bad for humans, animals, and computers. Smoke is very fine and sticky. It forms a fine layer on everything. It is much more difficult to remove than dust. Tiny and tight connectors are near impossible to clean. It cannot just be blown out. Cleaning usually involves some type of chemical bath with agitation. In sufficient quantities the fine layer can interfere with electrical contacts and create intermittent problems in connectors. It can also reduce the effectiveness of lenses in optical drives.
Some particulates are chemically active (acidic or basic) and will react with metals when there is enough ambient moisture. These literally eat through metals (corrosion and oxidation) and/or form metal whiskers. Corrosion and oxidation on connectors will interfere with signal transfer. In extreme and rare cases it will break connections. The metal whiskers are usually a problem around surface mount chips with high pin density and will short the pins together usually disabling or destroying the chip. Beware if your shiny computer starts getting tarnished for no apparent reason.
So what can be done to prevent these problems? The solutions are the same regardless of the platform (linux, windoze, mac, laptop, server, or even PDA's). Keep pets out of the computer room and brush them. Clean and vacuum regularly. No smoking in the computer room. Keep the window closed and run the air conditioner. If the room is bad, consider getting an air filter. Keep the computer at least a few inches off the floor (a dust and gravity thing). If it is really bad, get a 20" box fan, a 20" furnace filter (fiberglass type), and a couple bungee cords and make your own filter (these are great for catching pet hair).
Computers are still going to get dirty inside no matter what, so what can be done to clean them out?
Safety first. Any time you open up a computer, touch the metal sides for grounding. Static zaps can kill a component in a fraction of a second. If you can, leave the power cord plugged in and turn the power supply off at the back switch. This provides an earth ground with a real power off. Don't touch the chips nor fondle the parts.
A vacuum is a good start, but it cannot get into tiny areas and deep fins of heat sinks. As a warning, be sure to spray a little "Static Guard" (available in the fabric section of most major grocery stores) on to the brush attachment to avoid static problems. If you bang a brush attachment around the case too hard, you could also physically break something. I will normally start with a brief vacuuming of my parts to get the big stuff, though.
A high powered air blower usually does the best job. This can be from a real air compressor (but be careful of condensation) or the out hose of a shop vac. I recommend a cheap $25 shop vac from the hardware store called "The Stinger" and a corner attachment. It has a very high air output. You still need to be careful about about banging parts and breaking something. If you get the bright idea to use a leaf blower, you need to read this document again.
Warning 1: When you blow out a computer, DO IT OUTSIDE. The dust cloud that this can generate can be quite enormous. Make sure you are upwind from said dust cloud. Wear a filter mask if you have bad allergies.
Warning 2: Do not put a high pressure air stream on a fan without first preventing the fan from spinning. While the fan may sound funny and be exciting to watch, it will over spin the fan and damage the bearing and can also turn the fan into a little wind turbine and generate power that could be damaging to the motherboard. Insert a nonconductive pen, pencil, or stick into the fan to prevent it from spinning at all.
Warnings heeded, it doesn't take a genius to blow the dust out of a box. Make sure you get the connectors, heat sinks, and side cracks. If you have a floppy drive, pop open the door slightly and blow into that. Be careful with the power supply fans as these can often be hard to block.
How often you blow out your computer depends on your environment. If it is really bad, you should probably do it monthly. If it is really clean, maybe once or twice a year.
If your environment is really bad, consider getting that cheap/washable black filter for window AC units. It can be put around the computer in the various holes, but be very careful of not blocking air flow. I have done this before and it is very effective against hair. Just be sure to clean the filters on a regular basis and don't put them back in when they are still damp.