Computers tend to build up vast amounts of dust over a small amount of time (these PCS were hardly dirty) The dust tends to clog the fan vents & heat sinks of you computer. These are the areas you want to concentrate on with your can of Air. The best way to do this is to pulse the air spray as you sweep side to side, away from your body.
Below are some before and after photos (can’t see much because like I said they were not that dirty). I also uploaded a video for your enjoyment.
The can of compressed Air.
Heat sink for the CPU (this one was not dirty)
Enjoy the video below