Does the fan on your Compaq Presario always come on whenever you do anything even remotely CPU intensive, like gaming or video conversion? I always keep my PC fairly clean but I was missing one critical area: the CPU fan grate. Here's what you can do to fix this.

First get an old toothbrush, then open up your PC case and get your vacuum cleaner with the hose attachment. Turn on the vacuum and put it near the main CPU fan. Then use the toothbrush to clean the grate behind the fan itself. Just get it in as far as you can and a lot of dust will come out and be sucked up by the vacuum. Go around in a circle following the movement of the fan and use the toothbrush to clean the grate behind it the best you can. If you can remove the fan itself using a screwdriver then go ahead but it's probably not needed.

Now my fan never comes on. I thought I kept my PC clean but this one area was out of my awareness. Good luck!