One the bottom of the laptop there are a number of covers secured with screws. Its cover contains a certain section of hardware for your laptop. One might hold the ram, another the harddrive, the ram and cpu are usually close by each other. Grab yourself a can of compressed air. Unplug the A/C adapter and battery and make sure you are electrically grounded because there may be residule static electricty in the machine still. Careful remove one of the covers, exposing the inards of your machine. Follow the reccommended instructions on your air can and blow away any dust. Replace the cover, repeat as neccessary.
I've blasted air directly into the fan vent on an old laptop, but not sure how effective that actually is. The machine was old an I was testing the air can.