i got u, sometimes i start jabberin assuming ppl know what im talkin about.
the hsf (heat sink and fan) is the big hunk of metal with a fan on top of it somewhere on your motherboard. it sits on top of the processor (cpu).
Step 1: TURN OFF THE COMPUTER AND UNPLUG FROM THE WALL!
Step 2: unscrew the fan from the top of the heat sink
Step 3: locate the clip that is holding down the heat sink. a flashlight is usually helpful. it is usually a strip of metal that straps the heat sink to the motherboard. there is usually one side of the clip that is meant to stay stationary and a second end that comes on/comes off.
Step 4: get 2 sturdy flathead screw drivers. On the end of the clip that looks like it comes off, use one screw driver to push down on the end of it and use the other screw driver to pry the clip off the base. USE CAUTION WHEN DOING THIS! you do not want the screw driver to slip and gouge the motherboard. It will, however, take some muscle to get the clip to come off.
Step 5: remove the existing thermal paste with isopropyl alcohol and a cloth. if the computer is older, the paste is usually more stuborn. i have used a razor blade to shave off the paste from the heatsink, but it is important to try and not scratch the bottom surface of it. remove the paste from the cpu as well (the little black chip under the heatsink).
Step 6: use the artic silver 5 (from newegg.com or ebay) by applying a very small bb size amount of paste directly on the cpu. you can leave it as a ball, or take a straight edge piece of plastic and spread it evenly across the cpu. careful, AS5 is stringy like an extry cheese pizza. But remember, AS-5 is toxic, no eating it and wash your hands afterwards!
Step 7: carefully lower the hs back down in the same orientation that you removed it. (AFTER YOU CLEANED IT!! running it under hot water is ok, as long as you let it dry!) Hook the stationary part of the clip to the base and then squarely place the hs on the cpu. apply even pressure and use the 2 screw drivers to get the 2nd part of the clip onto the base, again requiring some muscle. I recommend a second set of hands to hold the heat sink steady while you wrestle with the clip. IMPORTANT: Do not allow the heatsink to slide around on top of the cpu after you applied the as5!! it will cause it to spread to the edges of the cpu. If you feel u moved the hs around too much, just take it off, remove the as5 w/ the alcohol and reapply.
Step 8: screw the fan back on (after cleaning it with compressed air or a damp cloth). Make sure it is blowing down!
If you have overclocked your cpu, take it back to stock speeds. the artic silver 5 takes ~100hours and 10+ heat cycles (hot/cool/hot/cool) to set up and be fully effective.
If this seems to compicated, you may just want to take off the fan and blow out the heat sink with compressed air. I would recommend doing this outside (its really dusty!) dust out the psu as well by sticking the nozzle into one of the fan openings.