This CNET forum post offers informative cleaning tips:

QUOTE:  I noticed that my computer box vents were grossly blocked by a blanket of dust. Even the back of my computer where all the cords are connected to it was covered with dust and spider webs. OK, I’ll admit I’m not a clean freak, and to be perfectly honest, I have never visually checked my physical computer up close since I had purchased it. I did some wiping on the exterior of my computer, but that only got me so far, as I could still see a lot dust on the fans in the inside. I did manage to buy one those canned air, but spraying it from the outside cleared the vents, but now all the dust is inside. I desperately want to open the computer up, but I’m not sure how to.

Several great tips can be found in forum posts attempting to answer this question.