****Warning do not use the 3M air duster without inhalant protection.************
Items needed 1. 3M Air duster 2. Vacuum 3. Anti-static Mat 4. Phillips screwdriver 5. Anti-static cloth or no frill
Step 1. Place you computer on a safe anti-static mat or a clean surface.
If you do not have a anti-static mat, ground yourself by touching the metal structure of your computer.
Step 2. Open the side of your computer using a phillips screwdriver, does not apply to dell.
Step 3. Disconnect the main power connection in your computer by gentle pulling it out firmly.
Step 4. Use the compressed 3M air canister and from a safe distance spray liberally at dusty areas. Start with the CPU fan as shown with a direct blast from the compressed air canister.
Example of a very dirty CPU fan.
Spray compressed air as needed but liberally.
Step 5. Use a no frill cloth or a small brush and the vacuum to get rid of dust residue from the case. 
Step 6. Spray other cooling fans, the fins and other computer components as well.
Step 7. Wipe of the outside of the computer with a no frill cloth or alcohol wipes. Then reconnect power and close the case.
Result is a clean CPU fan
Your computer is now spotless. Computer should be cleaned at least once a year.
Dell Dimension 3000 before cleaning ----------->Dell Dimension after cleaning
HP Pavilion before cleaning-------------------->HP After cleaning