To add to dkamm65, you should use compressed air. Do not use any vacuum cleaner (dirt devil, etc) because the static can damage the computer.
The biggest area to be concerned about would be your heat sink. It is the most crucial part and can also collect the most dust.
Also, if anyone in your house smokes, the computer fans will suck that in too and everything will have a nice layer of tar on it, which is just an added layer of disgusting fun when you clean.