I know the Unix based and NT based server,but how about Win2000??

NT & Win2000 .... is these the same ??

Windows 2k is considered the newest version of NT, its actually complete code revision so its not anything like NT, its much more stable, overall win2k is th most stable and bug free product i think theve released in a long time.

..and comparing NT vs. 2000. The win2000 is definitely the way to go. It's actually really quite stable and your ASP apps run far better in 2000. It also has better support for host headers

Okay maybe its a complete new code rewrite. But essentially the GUI is the same and you still use IIS to add/modify websites and the security settings for users and user permissions etc... are near enough the same. Two or three days max to migrate from NT 4 to Windows 2000.