Modern pc's need to breath. They can produce quite a bit of heat and this needs to be dissipated into the environment in order to prevent them from getting too warm and failing. There are a couple of ways dust can cause overheating. Pc's dissipate heat by moving air over heat sinks with fans. There are other methods of cooling but this is the industry standard. Dust can cause a break down in this process in a couple of ways. One is when it builds up on the heat sinks and prevents air from moving effectively. The other way is by building up on fan blades until the bearings on the fans begin to fail and it stops working all together. If either of these conditions occur the Pc will do its best to keep working but the heat will cause stress on the individual components.
Capacitors can swell up even burst under such stress. Capacitors are the things that look like little cylinders sticking up from the different parts of the motherboard and power supply. A single bulging capacitor can ruin the reliability of an entire pc in some cases. The ones that tend to fail most often are the ones near the CPU, in the power supply or on or near graphics cards. The thing all these places have in common is they tend to be places heat will build up if air is not moving though the case properly. Any capacitors that have domed tops or cracks and ooze on the tops have failed. Typically if this happens on your motherboard you will want to replace the entire computer system. Power supplies can typically be replaced fairly easily although some like the ones in Sony Desktops and some Dell pcs have special connections and dimensions that can make this much more difficult or expensive. Never open up a power supply to clean it or check it. That is the one part of a computer that can contain some dangerous forms of electricity.
Fans fail just because the dust throws of the balance of the blades and the extra weight makes the bearings fail and eventually this extra friction makes the fan stop working completely. Fans can be replaced often although the ones on graphics cards and chip-sets can sometimes be specialized and not easy to locate the correct size replacement for. CPU fans are fairly easy to replace and commonly available. The only issue is sometimes the plastics on older pcs can become very brittle once it has been heated beyond a certain point and removal of the old parts needs to be done very carefully to preserve some of the fan mounts. Once a chip has been heated beyond its tolerances it can fail or become permanently unreliable. So making sure the fans on your pc are working well is important.
Lastly we come to optical drives. These fail simply because dust ends up getting drawn into them and builds up on the lens. Once this has happened they need to be replaced. The dust can sometimes be removed with caned air but the drives rarely ever regain their full function and reliability. If you look at the front of your pc check around the eject button for the the optical drive. If you see dust build up there then the dust has already reached a level where it could become a problem. Fortunately almost everything that can be done with an optical drive can now be done with flash drives which seem to be more reliable in many respects.
Static electricity is specifically most damaging to pc memory. The newer types of ram DDR2 and DDR3 are much more sensitive to static than the older obsolete DDR and SDRAM from the past. They have much higher failure rates because of this too. Dust can create static electricity simply by moving large numbers of small particles past any raised surface. This is something to consider during cleaning as well.
Factors that make dust more damaging are large numbers of pets, cigarette smoke, metal based dusts from sanding and medical dust from dentist offices. Some of these require you to wear a mask and gloves to clean safely. I would always recommend people with allergies find someone else to clean their pc. Pc's should always be cleaned outside in a dry well light place. The best thing to use to clean most pcs is canned air. The most cost effective way to purchase it is often from places like Costco or Sams club. The prices for canned air vary from a few dollars for a large can to twenty dollars for a tiny can. I would recommend going with the less expensive air. Air compressors can also be used as a compressed air source as long as you make sure there is no machine oil in the air they put out.
You want to remove as much of the external case as you can from the pc when cleaning it . Most of the plastic and metal parts are removable from the case. You need to be very careful with older plastic cases as the plastic gets more brittle with age. You will want to check every nook an cranny on the case for dust and blow it out. Sometimes if there is a lot of dust and you have a newer optical drive it may be best to remove the drive and clean the pc and then reinstall the drive after the cleaning. You may also want to take out any ram from the motherboard and store it in a static safe container.
Its not uncommon after a heavy cleaning for a component to fail, this is caused by static build up on the part during cleaning. This is unavoidable once the pc has gotten past a certain level of dust build up. This may be something you want factor into this type of repair. If the pc is very old sometimes purchasing a new pc and migrating the data is easier and safer than cleaning the old pc.
Its also a good idea to wipe out the pc cabinet of any loose dust and dusting off your monitor can help it stay cool and last longer as well. People ask me often, if there is a way to avoid problems with dust and its not something that can really be avoided as much as dealt with on a regular interval. Every 6 months for a normal environment is about the right interval of time to check for dust build up inside the case. If you smoke, have pets, or work in a medical/industrial setting then you will want to half the time interval for each of these factors.
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