According to the FaQ in TW forums the problem is the user and not Steam itself.thehoff said:I have purchased the retail version and have had the exact same message pop up when trying to start the game. All I did to solve it was restart the steam client and then the game downloaded a small update and then played correctly. I think they have updated the steam client to solve this problem but obviously it hasn't solved it in your case.
http://shoguntotalwar.yuku.com/topic/46422/t/Official-Empire-Total-War-Support.html