Fans must blow out air to work efficiently and insulation has a funny way of keeping that from happening. After a new power supply and three cans of compressed air I could finally see the circuit boards again. At least that machine was only dirty. A more frightening experience occurred a few weeks ago. I opened the case and was greeted by a colony of spiders.
Computers attract dust and are a great place to hide for small creatures.
Clean it at least once a month. It and I will thank you. I opened the case and was greeted by a colony of spiders. They are also hard on computers. But many might be surprised by the number one killer of the all important machine.