I have a legal copy of Halo for my PC. But the Product Key Code on the back of the disk cover has gotten damaged. I can not read the first 3 characters in the 25 character code. I had to reinstall Windows Xp on my computer due to difficulties with installing service pack 2. Now I can not reinstall my Halo game. Is there anything I can do or do I have to go out and buy another copy of Halo?