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Disassembly it completely - even remove PSU, then vacuum it all.
Forget that BS about static eletricity - done that with hundreds of computers (and I do it regularly with ones I still have) and never killed one. Luck? I don't think so... Theory is good, real life experience is even better ;)
OFC you need to be careful using vacuum cleaner. Do not hit capacitors etc. Use brush tip.
 
An air compressor with a blower nozzle is the very best. Compressed air cans are okay, but they don't do nearly as good a job.
 
what 666sheep said is the true sure way to clean back to brand new. blowing canned air at the heat sinks won't do anything but blow the dust off the top of the heatsinks. Find a take apart guide for your specific model, someone here should be able to help you find that, if not PM me.

The powersupply collects an assload of dust, as well as the squirelcage fan that pulls the heat off the backside of the logicboard thats in the back by your hdd. The best way is to compeletly disassemble it and clean it piece by piece, its not really that hard at all 3 main parts (logicboard, cpus, powersupply) and the fans. I have and AntiStatic brush that I have from photography for lenses and it works great with brushing off the dust on the logic boards or boards in general where air won't take it off. After completely cleaning out mine the temps are a few degrees cooler as well since its getting clean fresh air not dust ridden air with carpet fibres and what ever other kind of particles in the air. I do it every 6-8mo just to keep things like they originally were.
 
Weed blower. Does an exceptional job.

Good for cleaning dust out of anywhere (eg car dashboard).

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Well, I'm a little bit concerned about cleaning it piece by piece because I'm not sure if I can put them back correctly or I can mess something up.
 
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