I've been through the forum, and while a few people have had the same problem, as far as I can tell no one has ever solved it.
I have installed Medieval, but when I insert disc one, while the start game menu boots up, it then tells me to insert the correct DVD-ROM and restart the application.
The disc is bona fide, it is definitely a problem with my PC, as I had exactly the same problem with Rome: Total War. I was unable to play it on my computer in that case, and ended up installing it on my brothers (newer) computer, where it worked fine.
My computer spec is suitable for the game, I am wondering whether it is a problem with my DVD drive, which is a DVD rewriter.
Would really appreciate any help, was gutted I could only play Rome when my brother wasn't, and now I've moved out, I might not be able to play medieval at all!