C:\Program Files\Activision\Rome - Total War

If you want with the default install that's where the folder should be

Then open preferences.txt

go to the third line down

SAFE_REFRESH:TRUE
USE_TRIPLE_BUFFER:TRUE
USE_WIDESCREEN:FALSE

And if widescreen is set to true that's your problem. I think this option is also selectable from the fraphical options within game. Anyways if that's not the case more details would be needed.