Every 6 months, or so, I take out my desktop machines and give them a through cleaning. Not just the outside, but the insides, where it matters. I hate to have the fans clogs up with dust and makes so much noise. Those dusts also get blown back to the room, making my allergies act up.
I take the computer to my driveway, set it on the ground, and open it up. I use a full can of compressed air to blow out the dust from every nook and corner. There are more build up on the fans and power supply areas. Every 6 months, I usually get a pretty good amount of dust to clean.
To get a good idea how much dust can accumulate inside a desktop case, this video is an extreme example of it. Phew, I can imagine myself sneezing my eyeballs out!