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Have you unpacked your data files? And if so, did you delete the two geography files? Those cause a CTD when entering a battle.
If that's not the problem, try adding an error log to see what the problem is. Go to your main Medieval 2 folder and find the file called medieval2.preference. Open it up in Notepad and add this to the bottom.
[log] to = logs/system.log.txt level = * error
Now when the game crashes, a new folder will be created in your Medieval 2 folder called logs. Open it up and there should be an error log telling you what happened.
"Religion is regarded by the common people as true, by the wise as false, and by the rulers as useful."