Probably just clean it out. Dust won't cause lag though, if that's what you have. Post more info though.
Overheating causes the fans to spin faster to combat the increased temperature. Dust blocks air flow which causes heat to stay in the chassis, and requires more from the fans to keep your components cool.
Ye, I just heard about that after you said it, I watched this video http://youtu.be/sZOHutci7aI. Off-topic: Are you going to take part in the competition in my signature?
Buy some compressed air, pop the side off the case and spray the dust off. The fans are adjusting to counter the heating up of the GPU/CPU.