Unless you can live in a DUST FREE zone, DUST BUNNIES will find thier way into your pc.
DUST BUNNIES are dust clumps made up of - hair, dead skin cells, fibres and if you smoke in your home sticky dust, made up of TAR and all the other nasty things, ( i smoke but not inside my house btw, so its not an anti smoke line )
Below are some ways to help reduce the build up of dust inside your case, and shortening the life of your pc components and helping to prevent overheating.
1) If possible try to have your pc case OFF the floor and on your desk, so as not to be sucking up dust / fibres from the floor.
2)Keep your wrok station (desk) clean and dust once a week ( If you have a girlfriend they are pretty good at doing this for you )
3)Try and buy as pc case with air filters, if you dont have air filters then make your own. (see my other post on custom air filters)
4)Clean your pc out once a month if you seem to get a dusty pc a lot, or once every 3 months if like me your room is not to bad.