I typically pull the side off my PC's and blow them out every six months or so. If you have animals and/or there are smokers in the house, that will clog up heatsinks and fans faster than you would imagine.

Get some canned air or take it out to the garage and blast it out with the air compressor if you have one.

Several years ago, HP was using some pretty bad thermal compound between the heat sinks and their CPU's. It would actually dry out, and become fairly non-heat conductive.

The best thermal paste you can buy and use has ground industrial diamond in it. I bought a tube of it years ago, and it lasts forever and lowers your CPU temps significantly as more heat is getting absorbed by the Heatsink instead of building up in the CPU.