Cleaning Computers and Other Electronics?

Pru asks: "What is the best way to clean out those old computers systems now caked with dust? The Slashdot community would probably be a great one to ask.. How should I clean out that old computer system? With dust caked on it and marks and scuffs all over the case; I would like to take a couple of the old systems I have around and put them back to work. But I would like to clean them up some first. My question is: How important is it to keep it free of dust? heat considerations, damaging the parts with cleaning and so on.. Maybe some of the veteran /.'ers that know some great advice and tips and tricks on this can help out the fairly new users here."