Firstly: When you first got it, it had no extra software loaded, nothing running in the background and very little stored on the hard drive. Now there are probably many games loaded, several programs running in the background and a clutter of rubbish on the Hard Drive. Also the Fans and heat-sinks are probably clogged with dust.
Run CCleaner and your Windows Defrag, and then get rid of all the gunge inside of it.
That may get you some of the way back to where you want to be.