You can't upgrade to IIS 6 without upgrading from Windows 2000 to Windows 2003 server. It is easy to think that Microsoft enforces this rule arbitrarily and could allow you to install IIS 6 on Windows 2000 with just a few bits of code changed, but that is not so.
IIS 6 is a complete overhaul and is not an "add on program" like Microsoft Office that can be installed on several platforms. It includes integration with new features, accounts, and drivers that do not exist in Windows 2000 and cannot exist without a sizeable overhaul of the operating system. The good news is that IIS 6 is a much, much better web server than it would be if it could be installed on Windows 2000. The bad news is that you have to upgrade to Windows 2003 Server to get it.
Reference:
http://www.iisanswers.com/IISFAQ.htm
