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Is it OK to use a vacuum cleaner to clean my PC? Compressed air just blows the dust everywhere, which is annoying. I've done it before and it never caused a problem. — William F.
Put the Hoover down. Slowly, now—no one's PC has to die today. Good, now we can talk:
No! You'd be better off building a diorama of Dust Bowl-era Oklahoma inside your case and presenting it at a county fair. Vacuum cleaners generate static electricity that could easily mean the death of your system. In fact, get that PC away from anything associated with vacuum cleaners—the carpet, for instance.
There are 'computer vacuums' which advertise anti-static features, such as the DataVac , but they aren't cheap. The cheapest and simplest way to clean your PC remains compressed air and rubbing alcohol, annoying as that can be.
Move your PC somewhere you can easily clean up later and use compressed air to blow out as much dust as you can. If it needs more attention, remove the fans and motherboard and carefully clean them with a soft cloth and 99% isopropyl alcohol.
Or, if you're really lazy, you could just dunk your whole system in a vat of alcohol. You probably wouldn't have a computer that works any more, but it would be clean. (Please don't actually do that. You might get away with submerging your motherboard as long as you remove everything, including the CPU, but that's probably not necessary.)
Update: There's been a lot of discussion about this advice, so I thought I'd elaborate. It is not the vacuum cleaner's power source that causes a problem. It's the static charge that builds up on the plastic nozzle. Additionally, vacuuming isn't effective unless you hold the nozzle very close to the electronics you're cleaning, so an electrostatic discharge is entirely possible. If you've been vacuuming the interior of your case for ages and have never had a problem, you've been lucky—a quick Google search reveals lots of people who haven't.
There's some good advice in the comments, too. For one thing, if you are blowing out the dust with the vacuum cleaner, you don't have to hold the nozzle close to the motherboard. You still need to be cautious, but it's a much safer technique. Nosterex recommends a leaf blower, which I think is hilarious and will definitely try.