How to detect all windows servers in network

My task is to detect all the windows nt 4.0 and windows 2000/higher servers in my WAN. How do I use nmap to detect only these two operating system computers. What ports differentiate windows nt 4.0 and windows 2000/higher. is there any way of diffentiating similarily between windows 2000 servers and windows 2003 servers?
Please help
Ankur Konwar