You can try to email Blizzard at [email protected] and explain them what the deal is. Maybe if you provide them with your CD key, they can send you a replacement CD. I'm not sure whether their policy on this has changed, but I know they have done it before.
However, that is only if you live in the US, I think, and you would still have to pay administration and/or shipping costs. It might be better if you would just go with what sjw7 said, and buy a new copy. Like he said, the expansion is cheap nowadays.
Blizzard will probably do that, but charge $/â‚¬10 for it which is more than the street value of the game and apart from the money, all that will cost you more efforts than a 5 minute shop visit, I guess.
Buy a new game and, if you have WinXP or a second computer, the classic game as well. That way you will have two key sets, so you can start the game twice and do things like quicker and safer muling, self-rushing, having a Hellforge dummy in the game while questing, making MF runs with a combo of two characters etc. I did that and it added a lot to the fun of it.