Don't know how to use a "crack" for pc g

Well, I bought call of duty world at war and installed it. Within weeks, I lost the cd and can't play the game without it. I heard from a friend that you could download a "crack" for the game so you could play it without the cd. So, I downloaded one and when I clicked on it, it said something like " you need to install the game again" I don't really know what to do, so could somebody tell me how these things work?