As Vaanish mentioned, if this is a pre-built from a retail company, like HP or something, you won't be able to overclock.
If you are, my guess is you won't go far. Chances are, you have an old motherboard. You should tell more specs about that system. Like, what graphics card? Speed of RAM?
I guess if you can overclock, you could probably get at least an extra 100MHz-300MHz, out of that P4, considering the motherboard and cooler. Which I'm assuming, you're using the stock. But you won't see a significant boost, from a 300MHz overclock. The graphic card would be easier to overclock though. And you can do this with software. The popular ones are, ATI Tool or Rivatuner.