there's a couple of opinions on the 'best' way to clean out a PC case. One is to open it up, strip it apart and use compressed air to blow away the 'cob webs '. The other (which I'm an advocate of!) is to use a vacuum cleaner....this however, while extremely quick does attract a few comments about the risk of static build up.
Dust on fans is notoriously difficule to clean off. You have little choce but to take the fan out and use a cloth to rub the dust off of the fan blades.
It's an easy job if you have good dexterty, but take your time and make sure you have a suitable earthing point before yoiu start.
All that said, prevention is better than the cure: