A lot of people without knowing what they are doing will also stick their computer in a cubby hole in the computer desk. This is absolutely the wrong place to put your tower.

I have in the last 4 months cleaned two of these. One was an emachine, and after clearing the dust out found 5 blown capacitors. The other was a Dell and it was just clogged to the hilt with dust. Luckily, no blown caps. Both computers belonged to family members and I routed them out and explained some things to them. The emachine has since been replaced with a build I did for them.

Btw, when using canned air, do so in small bursts of air. This way the can doesn't get cold.