Why does my computer overheat?

I bought my computer about 2 years ago, more or less... I used to have no problems with the temperature of the computer. I play games on my computer, one of them is World of Warcraft. I used to have no problems with the game and my computer would run it without overheating or getting too hot. Now everytime I play the game the computer gets extremely hot causing lots of framerates spikes and other issues. What can I do to fix my computer so that it doesnt overheat anymore and it stays cool? By the way I already bought a cooler and already got it cleaned at a best buy store. Please help.