If this is a prebuilt machine you wont find it. Companies like Dell, HP, Compaq and the rest remove these functions from their BIOS so that people cant overclock their machiens. Unless you have a machine like the Dell XPS which is built to overclock and comes factory overclocked.
With a machine like you are using. I can almost garantee you that you cant overclock it. I dant Overclock my dimension 8400. Just the way it is.