Here is an old post about this problem, and the solution was to fax a picture of the box to CA and they sent him a new cd key.
Today, I'm going to check a couple of shops that sell used pc games, and if I see the Viking Invasion Expansion Pack or the Medieval Battle Collection, I'll buy two of them and send the cd keys to the guys who got invalid cd keys with their Medieval Gold. There are a couple of used VI expansion packs listed on amazon for $8 + $4 shipping, so that's a possiblilty as well.
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