Like PCs, laptops use fans to blow air on components that would otherwise overheat. Typically a laptop will have one or two fans cooling the main processor and graphics chip or chipset. Many designs use a so-called heatpipe system to draw heat away from these components, with a single fan cooling a heatsink which is attached to the heat-pipes. The heatsink will have lots of 'fins' to dissipate the heat, and dust can also build up between these fins, reducing cooling power enormously.
What you need to know is that the fans in a laptop are even more critical than in a PC. With everything packed in to make the laptop as slim as possible, there's very little room for airflow. And that means that even small amounts of dirt and dust can hamper a fan's effectiveness.
The first signs of a clogged up fan are random Windows crashes as the system overheats. Or, if the vents are blocked with dust rather than the fan blades, you'll notice the fan running constantly or at a high speed than usual.
You can try various techniques, with the easier methods having a lower success rate.
Method 1:Blow out the dirt
If you're not confident using a screwdriver to dismantle your laptop, grab a can of compressed air or try a camera lens air duster if you have one. Vacuum cleaners are not recommended: they're quite ineffective at removing dust and they can also cause a build-up of static electricity which you don't want around a laptop.
Obviously make sure the latop is powered off before you start. Insert the nozzle of the air duster into the vents (or as close as you can get it) and start blowing the dust out. Try and stop the fan blades turning using a long plastic object - you'll have a hard time removing dirt from a spinning fan.
Method 2: Open up the laptop
For those that are happy to wield a screwdriver, turn off the laptop, flip it over, and remove the battery. If there is one, remove the plastic or metal panel in the base and hopefully it will reveal the fan. Your success here will depend entirely on your laptop model. If you don't see the fan at all, you'll have to go one step further and remove other parts. There are many YouTube videos that show how to take laptops apart: just search for your make and model.
Now it's time to blow out the dust, something you might want to do outside. The best way to do this is with a can of compressed air. If using compressed air, press the button in short bursts from lots of different angles. Don't get too close with the nozzle and stop the blades moving as per method 1. You'd do well to unscrew or otherwise remove the fan at this point and check for any other dirt clogging up the heatsink or air vents.
After you've blown out all the dust, replace the panel and battery and you should find that the fan runs much quieter when you power the laptop on again.
Method 3: Replace the fan
The plastic bearings in laptop fans don't last forever and if your fan is emitting a loud whine (which it didn't when you first got the laptop) it's probably time to replace it. Here, ebay is your friend. It helps if you have a popular make of laptop: we found a lot of Dell, HP, Acer and Asus fans but you'll be able to search for your exact laptop model.
It helps if you have your laptop's fan for reference, and check with the seller if the photos are of the actual item if you're not sure.