See the thread below (terrible gaming performance thread) but to summarize flip your laptop over and remove the two fans nearest the hinges (2 screws each). One will pull out easily while the other one (where the headphone jacks are located) requires you to remove one more screw (maybe 2...looks like I was missing a screw) and pull up on that metal hinged bar). I had tons of dust built up in the metal thingies next to the fan (you'll see what I mean). You can wipe most of the big stuff off and blow off the rest. Do the same for the fans.
Just cleaning those 2 made a difference for me (i.e. the fans ran quieter). The third fan requires removing the keyboard; if you've installed the MR9800 video card, then you know what to do. If not, get ready to pry off some plastic! (check the dell support site for the manual for the 9100...or check the link in the thread below).http://support.dell.com/support/edo...ard.htm#1000473