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Clear out the dust
Dust build up over time can impede airflow, and airflow is vital for keeping system temperatures down. If your system overheats, it'll likely throttle its performance down to cope.
Cleaning out the dust is easier if you've got a desktop rather than a laptop — you can still clear the dust away from vents in the laptop, but be wary about opening it up to do a thorough clean, as depending on the vendor this may invalidate your warranty. If you're out of warranty, and you're confident of navigating the maze in most laptops, go for it.
The first step is to remove general dust from around the system. You could use a moist paper towel and cotton buds to get into harder-to-reach areas, but one of the best tools you can employ is a can of compressed air. Make sure to avoid vacuum cleaners — or at least getting overzealous with them. We've known people to have sucked capacitors right off the board. There are other issues with using a vacuum cleaner, too, as Brian Cooley of CNET tells us:
...you might be tempted to stick a vacuum-cleaner hose inside and suck out the dust. Don't. Vacuums create static electricity, which is deadly to sensitive electronic components.
On that same note, don't be tempted to reverse the flow of your vacuum and blow the dust out of the computer. The dust inside a household vacuum can be harmful to your health, and you'll be spreading it all over your PC. Also, you risk blowing out sizable particles, which could physically damage internal components, especially if you're using a workshop vacuum. The beauty of compressed air is that it's clean and particle-free.
Before you start blasting, unplug your computer and take it outside — or at least to your garage. Now, working from the top down, blow out all that dust (put on a dust mask, unless you want a face full of grime) ... be sure to spray air in short bursts, keeping the can upright and the tube at least a couple of inches from the hardware.
Next, you'll want to get your fans and heatsinks clean. Cooley has some tips here, too:
Start by powering down your PC, removing the case lid and locating the various fans. Starting with the power supply, blow through the internal slits from inside the chassis, aiming so dust will exit the back.
Next, blow into the intake fan (if there is one) to push more dust out the back. Finally, blow the blades of the rear exhaust fan clean. If possible, aim just beneath the centre, where the motor meets the fan assembly, and blast again. Repeat the process for each fan, keeping the can upright at all times.
Now restart your PC, and while the fans are spinning, spray them once more — very briefly — to really send the dust flying.
Reseat the CPU and GPU heatsinks
This is more applicable for the desktop, but the same basic principles apply for a laptop.
There's a small possibility that the thermal conduction between your CPU or GPU and its heatsink isn't optimal, causing things to overheat. If you want to make sure that things are fine, you're going to need some isopropyl alcohol and thermal compound.
Firstly, make sure that the heatsink is attached via mounting holes to the circuit board, rather than directly to the chip. If it appears as if there's no obvious way that the heatsink is held down on the chip, it's using thermal glue or thermal tape to form the bond. If this is the case, ignore and move on, it's unlikely these are causing you issues.
After separating the heatsink from the processor, you'll notice some goop that was last used as a thermal interface. Clean it away by applying a small amount of isopropyl alcohol to a clean cloth and rubbing it until it's gone. Apply a thin but consistent veneer of new thermal paste across the top of the chip (application can be made easier by using a scalpel or old credit card to spread the paste), then reapply the heatsink.