If your computer is running slowly or making a lot of noise as it tries to work, the problem could be dust. Your system's intake fan pulls dust in with the fresh air that cools the internal components. The dust then settles on the parts in your computer, which in turn, traps heat. If left unchecked, this can result in overheating. While you can't entirely eliminate this problem, you can certainly lessen the impact by routinely dusting your PC.
Read here for other reasons your computer might be running slowly.
Depending on your environment, you need to dust your computer every six to twelve weeks. Some of the things that contribute to having more dust are pets and construction near by.
Use canned air to dust your computer. Canned air is easy to use and is available at most computer and office supply stores. Don't use a vacuum cleaner. A vacuum can create static electricity that can damage computer components. A vacuum can also create enough suction to make your fans spin hard enough to damage them. Don't use a feather duster or rag, either. Rubbing components can damage them.
Use the owner's manual for your computer to see how to open the case. If you have a laptop, power off and unplug your computer, then direct the canned air into the vents at an angle.
Turn the power off and unplug your computer. Open the case and take a look, if there's a lot of dust, remove the largest chunks by hand.
Follow the directions on the can of air and hold it upright when spraying.
Spray the front vents, fans, CPU cooler, graphics cooler, heat sinks, power supply, rear vents, hidden/concealed spaces, and any other place where there is obvious buildup. Continue to spray until all of the dust is gone.
Close up your case and reboot. Upon rebooting, you should notice a difference in the way your fans respond. In most cases, they will spin faster, keeping internal components cooler over time. Learn more about the internal components of a computer.
Cleaning your computer can be a bit messy; you'll want to vacuum up the dust you sprayed out of your computer. But your computer will run better and you will prolong the life of the components.