A Question About A Half-Life Game?

Well, basically I purchased a used Half-Life Generation game online and I just got it today. I managed to get it installed but but before I could start the game, it says "Please type in the CD Key displayed on the Half-Life CD case". There's some sort of a long number as well as several letters on the case and I've tried it along with several other numbers found on the back of the case but none of them seem to work. I really need to know where I could find the CD Key. Perhaps the chap who previously owned the game had the tiny booklet containing the key. However, I'd really, really appreciate it if someone could please tell me where or how I could find the CD Key. I'd be really, really, really grateful, you've no idea how much!!A Question About A Half-Life Game?
certain cd keys have letters that look the same as numbers. check and make sure that you typed in the correct letter and numbers. I once had a game where i typed in a B and it wouldn't accept the key because it was actually and 8 but it looked like a B.