It's amazing how much dust and debris can accumulate around the vents. Remember to not just clean the fan but also all the vent slots. In a desktop PC, there are usually slots on the lower front edge, normally hidden from view by the case, which are relied upon to draw cool air in over the motherboard. You want to make sure you get those clean for best airflow.
You can buy cans of compressed air which is handy for blowing out dust from awkward corners, especially considering that brushes with man-made fibers are not the best tools around static-sensitive components.
