I'm sure that the cd isn't an expansion, and I'm sure that it's genuine!
I can install it, but I can't just play...

About the last option, I think it's the right way, because before this original software, I tried to install another one... without success.
I think there is something on the pc of that 'copy' (also if I think I deleted all files of that one) that is still on the pc and doesn't let me start the game.

(I think it because, now that I think on it, I had the same problem with Warcraft: I installed a copy version, and when I tried to install an original one, it said me to insert the right cd...)

So, how can I do?

Thank you very much.