i repaired a pc for a guy a month or so ago. he got it in 2001 and had never cleaned it. he has a dog and 2 cats, i have never seen so much crap inside a pc case lol it even had a dead spider in there. i had to take each part out one at a time and used two packs of wipes and a tub of cotton buds. just as well i charge by the hour

best way to do it is to make sure the power lead is unplugged first. use screen wipes for the non electrcal surfaces and cotton buds to clean out the fans, heat fins etc. clean your filters weekly and dont put your pc on the floor if you have cats or dogs
dont ever put a hoover inside the case, bad things happen dont they stealth! hoovers are bad for static discharge