Even in the cleanest of domestic and commercial environments, a computer will manage to attract dust. A fair amount gets blown through the system as can be seen if you look at the wall or partition behind the case. Some of the lighter particles travel up with the warm air and are caught on the wall just above the case. Heavier particles fall to the floor. Go take a look, and remember that this is only the stuff which escapes all of the catchment areas in the computer case.
Airflow should enrter in through the front and lower sides, and then vent out through the back and upper surfaces. What happens is that dust enters through every little crevice, the gaps around DVD drives, through disktette drives, and especially through the gap just under the front panel. Until these areas are cleaned out, there is no point in going further back through the case.
The major dust magnet is the CPU fan and heatsink assembly. When this blocks up, your computer will crash. Shown here is a standard Intel assembly. Air is sucked in over the top of the fan and blown through the fins of the heatsink. To clean it out, you should loosen the dust with a wooden cocktail stick and then use compressed air to blow through the fins and back up through the fan, and pick out the dust bits with the cocktail stick.
For PCIe video cards, the picture does not look so rosy because PCIe slots are very often in line with PCI slots, and spaced about the same distance apart. If you have a couple of PCI cards installed in the average system, you will have used up all available slots and the one nearest to the video card will hamper the free ingress of air into the fan intake. Needless to say, this fan will eventually clog up and cause overheating of the video card. This may lead to odd behaviour by the graphics system, and will always help with overheating within the case.
Power supplies always suck air OUT of the case and do it slowly, so dust doesn't get blasted onto the surfaces inside it. The secret again is to blow the dust back in the opposite direction to how it got there in the first place.
Hard drives get hot too, especially if they are mounted close to each other. This is a problem with old cases which were originally designed for slow spinning hard drives which had a large surface area. Old drives didn't get hot like the new high speed drives do, so if you are going to fit multiple drives, get a new case too.
Air is drawn in through a grille on the base, goes through the fan and is ejected through a vent in the side of the case.
To clear it, blow air in through the side and then pull the dust out of the fan with a wooden cocktail stick. If you can't see the fan as in the photo, all you can do is hope that the dust breaks up enough to be ejected when then the computer is restarted.
Internal design is sometimes lacking with laptops. Fortunately, they tend not be be used or left in dusty areas, so the problem of dust build-up is far less common. As long as they are used on a flat hard surface, or a 3rd party cooler base, you may never have to clean the computer out.