NT 4.0 and Win 2K were more than simple DOS boxes with a fancy shell. I'm not sure about NT3.51... as I never really had a chance to toy around with it much (at all, really). I only got to use DOS, 3.11, 95, 98, NT4.0, ME, 2K and everything after. Windows XP was the first OS to exclusively use the NT kernel for both business and home users... but NT was a different animal altogether compared to DOS or Win 9x.