As was suggested above you may need to go buy a can of compressed air, remove the side panel and blow it all out. If you have never done this before, you might wanna take the pc outside to do it. You'd be surprised how much dust can collect inside a pc. Make sure you blast that fan out good too.
As Bob said above you might have too much stuff or processor intensive stuff running that is causing the processor to overheat. Might be trouble down the road.