Is there an error message after the CTD? If yes, please post it here. If no error, does the crash happen repeatedly in the same spot in the game? If yes, than your VRAM may become saturated and you need to reduce some settings, like AA. If you are using any combimation of mods it can also cause a crash so you need to usinsall them all and then install them one by one to see where the crash is coming from.