Just goto some computer store like Compusa or something like it. Just put up a ATX Power Supply. Just get a 350w one or a 400w one. Then take the 3 screws out then take the old Power Supply out. Then just plug in the new one to the main board then your hards and other stuff then just screw it back in. That's it. Just make share you get a ATX Power Supply not any other type.
Hope that helps