If your harddisk is full it makes it hard to defrag. Try freeing up some space by moving stuff onto a backup/external drive. Then try the following to troubleshoot to if it is a hardware problem or software problem.
Checking your Hardware
Yes, Physically cleaning your computer can help reduce the heat build up. If it is a laptop you can buy a fan to place under your laptop to cool it down. Check to see in your task monitor if you are using alot of RAM or cpu cycles. If you are using up a large percentage of your RAM you probably need upgrade to more RAM or use less programs at the same time. If you CPU is continously spiking, it could be your computer is overheating or you have a program that using it. You can try to find and kill it using your task monitor.
Cleaning Up Your OS
1) run chdsk on your harddrive to see if you have any bad sectors
2) try degfraging your harddrive
4) if using a lot of ram try uninstalling some of your unused software
You can try some software fix some of these problem if you dont want to do it manually.