Compressed air is the way to go - or if you have a compressor, use it on a low speed.

All fans - the ones on the back of the tower, spray from outside so that they spin REALLY fast. I also spray the vents in the power supply - the large metal box that is located inside the computer; on the outside is where the power plugs in.

Here's a good YouTube I found on dusting out a computer.

http://www.youtube.com/watch?v=8QZ8j...eature=related

About a year ago, I had two clients who were terrified that their computer was toast! It wouldn't run at all. I pulled off the side...I swear there were cobwebs there and a TON of dust. Cleaned it all out, and it ran perfectly!