I had the problem solved with the advice you guys gave me. I didn't understand exactly what Fable meant - not Fable's fault, his explaination was clear, it's just that I am a computer noob... - so I asked my brother, who knows a bit more about computers. After being very busy for some weeks, he got the time to give it a look, and I showed him this thread. I think he did exactly what Fable suggested, and it worked out fine. Both Shadow's of Amn as Throne of Baal are properly installed now, and play like normal.
So thanks for the help everyone
