Erasing hard drive

I just upgraded my PC to a new one & I want to give my old one to a couple that cannot afford to buy one. My question is how do I erase my hard drive & then re-down load I have win-XP pro with the disc that came with it. I cannot figure out how to erase all my personal data (things) without buying something else like a wipe disc or something. I hate to spend more money on something I am allready giving away. Help!!!!!!!