When you get it cleaned out, think about where you place your tower. You should place it on your desk or some place high up to avoid dust. Having the tower close to the ground just draws in dust from the floor and carpet.
A machine I was asked to look at once was totally disgusting inside. Everything was dripping with a black sticky goo and it smelled worse than it looked. The owner of the machine was a heavy smoker and he kept his ashtray immediately in front of the air intake grill at the front of the computer.
I put the side back on and told him there was nothing I could do.