Invalid key code to reinstall Windows XP

I tried to reinstall windows xp with the original disk I purchased that had a corisponding keycode. I was unable to load it as the keycode kept coming back invalid. I tried to contact microsoft whom sent me to compaq the provider of the disk when I purchased the computer. They were unable to help without subscribing to three days of technical service. Can anyone help me to get a valid keycode to reinstall without having to pay for a service that should be provided with the cost of disk and computer.