I remember the days of NT4 and still have my workstation & server disks, IMHO i think unix/bsd is much more reliable/stable than NT4. Windows NT4 (server&workstation) when not rebooted regularly weird things always happened, like sticking start menu's and unresponsive mouse clicks. Even though network services would still be operable (dns, dhcp routing etc...) the gui would always seem to develop issues the longer the uptime. Windows 2000 Server was much nicer than NT4.
using OpenBSD on the other hand the uptime was years with 0 issues, with rock solid services uptime & lightening fast operations.
A good Linux server that i've found to be very reliable/stable/secure with many server features is SME Server. SME takes 10 minutes to install, including all networking services like routing, firewall, mail, spam assassin, virus scanning, web, samba, afp and a load more & also is all managed via web interface.
Leopard Server is also very nice with almost identical services to SME right out of the box but with a much prettier interface

take your pick