Hmm... I actually lost one of my CD keys as well. Luckily my brother has a copy of the game, so I can use his if I ever need to reinstall. I think contacting Codemasters might be the way to go, as has been said. Or give them a few more bucks for another copy... you know they deserve it
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In the future, I'm going to write the #$(% thing down on the CD itself, because obviously I can't be trusted to keep the book and the CD together at all times...