Take it outside and hit it with compressed air from a air compressor (not the wimpy cans) - the dust will leave the computer in a large cloud. This is the easiest, and most thorough way to clean a computer. The dust does not get blown around to other parts of the computer, as it becomes instantly airborne. Be careful not to over spin the fans from the turbine effect from the compressed air - you can easily kill the bearings, and possible fry the electronics in them from excessive voltage generation, and in a worst case scenario cause shrapnel from the fan flying apart from centrifugal forces.