When do you experience CTD? Is it during battle? End turn?
I was experiencing a LOT of CTD at end turn or when I saved a game. When I upgraded my hard drive to a much newer model the CTD's disappeared instantly. Who can say whether a reformat would have done the job too, but it was certainly some sort of corruption as it was trying to save the game to the hard drive.
I occassionly get crashes now in battle (usually seige) but I think that is because I have overclocked my graphics card and my PC sits behind my couch without good airflow.
Could be any reason mate, just give us a little more info.