For the last month or so, my wife's laptop has been noisier than you'd think possible for such a thin, compact PC. The culprit: The cooling fan, which seemed to run non-stop and at the highest possible speed.
If you've ever opened up a desktop, you know how much dust can get sucked up in there. Laptops, though seemingly sealed up tighter, are just as susceptible. I could tell without even looking that dust was clogging up the fan and the space around it, causing it to run loud and hot. It was only a matter of time before the machine would start overheating and locking up--or worse.
Fortunately, this is incredibly easy to fix. All you need is a small screwdriver and a can of compressed air (or an air compressor).
Power down the laptop, flip it over, and remove the battery. (Unplug the AC adapter, too.) Look for an air vent on an outer edge of the laptop; there should be a nearby access panel on the bottom. Unscrew the panel and remove it. You should see the fan right underneath. (Your mileage may vary, but the three laptops I have here all have panel-accessible fans.)
Now it's time to blow out the dust. (You may want to do this outside.) Hit the fan in short bursts from lots of different angles, making sure to blow most frequently in the direction of the air vent. If you're using an air compressor, as I did, keep the pressure relatively low, and don't get too close with the nozzle. You don't want to damage the fan or anything else, after all!
After blowing a fairly substantial amount of dust out of my wife's laptop, I replaced the panel and battery and powered up the system. Result: Quiet as a mouse, just like it used to be. If your laptop is more than a year or two old, it's probably overdue for a similar cleaning. (Same goes for your desktop.)