Find an opening in your case, or an air hole on the bottom. With the PC running, by placing a match or lighter flame close to the hole or opening you can check if the air hole sucks the flame toward the case or does it push the flame away. If the flame is pushed away then this indicates that you have positive airflow within the PC’s case and that will keep dust away from accumulating inside the case. Well if the flame is not pushed away but rather sucked in well i guess it’s time for a case upgrade.
Now with the dust accumulating inside the case even with the said “tested” positive airflow, it’s still prone to dust accumulation in time, which can lead to overheating and other problems. It’s best to periodically, take the PC outside, remove its cover, and with can of compressed air from 8 to 12 inches away to blow out the dust, and also clean out the fans too…







It has happened with Vista and is seemingly also being built into 
definitions.