need help retrieving/changing cd-key

[treetagx1000]

hi im new here..but im also having a problem..somone is using my cdkey and i cant reinstall the game due to various reasons but i do have another cd key..i was wondering if anyone could help me change it..i heard from a friend its stored in one of the .mpq files but i dont know whitch one and i dont know how to change it! just wondering if anyone can help..thanks