Another thing you can do is shut down the computer, unplug it, open the case and take a look inside it. If the CPU cooler and heat sink and other components are caked with dust, get a couple of cans of compressed air from an office supply store (Wal Mart usually has it too, in the electronics department) and blow all that crap out the case and off your components. (Note: do NOT use any kind of shop or industrial air compressor; those create way too much air velocity and will destroy components). Make sure your CPU cooler and chassis fan connectors are snugly connected to the motherboard. Make sure all vent slots inside the case are clean and free of caked dust and other debris. Plug the computer back in, power it up and make sure all of your fans (chassis, CPU, GPU, PSU) spin up.
If it still overheats, you'll need to replace the CPU cooler.