Hi Peter - the best way is to buy a can of "air" sold under different names. Take you computer side panel off and take it outside. You'll be surprised how much dust accumulates in there. Blow not only the fan but the aluminum heat-sink thats under it until it look perfectly clean - several different angles help. While you're doing that blow out the dust from the fan in the back of your computer power supply. Same thing - blow from the outside in and through the grill work on the inside towards the outside. Finally blow out all the dust from inside your computer cabinet.
You can plug it in and see if the fans work properly before you put the cover back on. Don't blow air while they're running!
If you have access to a compressor that will work but watch how much pressure you blast them with. Hope this helps. One last tip, if your computer tower sits on the floor, put a piece of cardboard under it so it doesn't pick up lint from the carpet or floor. Overheating is one of the major problems people run into by not doing this simple maintenance. Hope this helps. If you have any questions feel free to contact me. Bruce (shopbruce@gmail.com)
Bruce
June 2008