My PC's FPS is so aweful

2 months ago, I play Pro Evolution Soccer 6 on my PC just fine. I never once encounter any FPS issue.

But now, for some reason I'm lagging very badly. This is a single player game so no lag is involved.

I've cleaned up my PC from spy/adware, defrag it, scan it, optimize it etc etc. When I play the game, I made sure to have nothing running in the background other than the essentials. Why is my PC performing so badly?

Also, this isnt the only game I've issue with.. but this one is the best example from my experience because I play this game regularly.

Short of formatting, what can I do?

 

Actually the above situation was when I left my PC on for 5-6 days straight. I restart it a few times however...

But last night I turned it off and today, my PC works perfectly fine. This led me to believe that my PC was overheated..

How do I check the temperature of my PC if possible?
So what do I need to replace my PC with? the fan?
Is there a webpage that shows you how to take out the fan and replace the new one?