Rojo is correct, this could also be due to a fan failure inside your PC case. With the PC turned OFF, open up your PC case. When it is opened turn on your PC and Observe every fan in there. Focus on the PC fan that is on top of your CPU and also the one that is inside of the power supply. But there are probably several other fans inside your case that could be dead, so look everywhere for a dead fan.
A second thing Rojo hit on is cleaning the inside of your PC. Heat is a huge reason most PCs overheat. While the PC case is open grab a couple of cans of compressed and blow away all the dust. As the can us blowing/dusting the inside of the case switch out to a new can after the one you are holding gets too cold.
If you don’t know what compressed air is here is a popular brand on Amazon: http://amzn.to/1AixIm4