1) Migrate your IIS settings to the Windows 2000 Server: http://www.microsoft.com/serviceproviders/downloads/migration_toolsP65238.asp
2) Migrate SSL certificates and user accounts, if necessary. During this step, also migrate anything that the Web server requires but the migration wizard didn't get, such as log-analysis or log-rolling tools, content management systems, and databases.
3) Bring down the NT 4 Web site.
4) Copy the files and permissions over to your Windows 2000 Server. For detailed instructions, refer to KB article 174273, at http://support.microsoft.com/?kbid=174273.
5) Bring the Windows 2000 Web site online.
6) Test the Windows 2000 Web site. If you can't get it working within your scheduled downtime Window, bring the Windows NT Web site back online and try again during your next maintenance window.
7) If Windows 2000 is doing what you want, either configure Windows 2000 to use the IP address of your Windows NT server, or change your DNS so that host records that formerly pointed to the NT server now point to your Windows 2000 server.
For more detailed information from MS, check these links out:
If you must minimize the downtime, you can keep the NT Web site up while you transfer content. Just be sure no updates or transactions can occur during the transfer. Depending on the nature of your Web application, this might be difficult. Basically, you don't want someone placing an order on your NT Web site during the migration process that doesn't get copied over to the Windows 2000 Server. Good luck.
This was first published in August 2004