It’s bad to clean the inside of your computer with a vacuum cleaner because vacuuming creates a large static build up that could (and most likely will) discharge into the sensitive electronics inside your computer case.
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What we’d recommend doing is taking your computer case into a well ventilated area (outside on a sunny day or in your garage is a great place), grounding the case to protect against static discharge (although the risk here is very very low) and using compressed air to clean the dust off. If you’re using an air compressor (as opposed to just a can of compressed air from the computer store) make sure to start a good 24″ or so away from the case and work your way in closer. You want to use just enough air pressure to blast the dust off the surfaces and out of the case without overdoing it and pushing dust into even more difficult to remove places.
One important thing to consider: compressed air (from a compressor, not a can) contains minute amounts of water vapor. Although we’ve never actually heard of this happening to anyone it is (however remote the chance) possible to blow moisture into the connectors on your mother board and damage it if you were to boot it immediately afterwards. This is in the range of lightening-strike remote, however. None the less to be extra cautious we would recommend that you leave the computer off and in a warm dry location for a few hours after you give it a good air-compressor cleaning to allow any residual moisture (if it’s even there to begin with) to evaporate. This borders on paranoid caution, mind you, but better safe than sorry.
