It says that Medieval II is not installed. Anyone else have this problem?

For some reason it says that Medieval II is not installed.

I get around that by giving both sides a number of wagons/carts to represent camps. I stick them off in the corners of the map so that they don't get in the way of the game, and they boost the unit count sufficiently so that an army really has to be thoroughly wrecked before the game ends.

How do you go about getting the old version?