You'll want to go in BIOS to underclock your CPU, and use Rivatuner (program) to underclock your graphics card. Usually the BIOS can be accessed by hitting tab/delete/F1/F12 (depending on your motherboard) when your motherboard logo flashes on the screen (you'll also see which key you need to hit then as well). Then you can usually find a tab that will allow you to change you "FSB Frequency". Dropping it lower will underclock your CPU. I suggest going down by multiples of 10, but don't go under two thirds of what you're currently at.
Unfortunately, if you have a pre-built PC (meaning, you got it at a store) your BIOS will usually be locked and you will not be able to adjust your hardware settings (though you can still use Rivatuner to adjust your GPU). Reason, It's an "integrated video chipset", which means it's built straight into your motherboard. There's no way to underclock it either.
But I could be wrong.
But it sounds like you might be making a trip to your local computer repair store.