I realize most people discourage the use of a vacuum to clean inside a PC, but I have been doing it for years. I have a shop vac and I found this 1-1/4" Micro Cleaning Kit - Shop-Vac Accessories
mini-tool kit and it works great.
I find pulling dust and dirt out instead of blowing it out into the room to be better.
Bill's right, if you use compressed air on a fan it can exceed it rated speed and destroy itself, stick a finger in the blades so it won't spin.