Not sure how much more i can noob it, but ALL cpus have a power saver mode, called intel speedstep, or amd cool n quiet.
The cpu slows down and bthe computer uses less power. In the BIOS, the setting for most intel chip boards is called c1e and something else.
Now, as you have a prebuilt computer, its going to be next to impossible to overclock. Theres windows ways to do it, but i don't trust them, or know about them.
Because your systems old, its going to heat up fast, so if you do try it, becareful and watch your temperatures or you'll fry your computer.