Command and Conquer Generals Zero Hour redemption code?

Hi,

I bought the original Command and Conquer Generals Zero Hour expansion pack years back (I believe in 2003) and I want to re play the game. However, one of the discs for installation (disc 1) is missing from the case and was wondering if it is possible to download a soft copy of the game instead using the same serial key from the box.

I already have the first Generals installed on my laptop. Hope I can revive the game! 

Thanks!

Nike