How do I determine what version of IIS is installed on Windows 2000 server

I have a Windows 2000 advanced server acting as a web server. I need to install a Premium SSL ID from VeriSign and the installation instructions are different depending upon what version of IIS I have installed (4, 5, or 6). I have spent about two hours scouring the Web and looking at the components of IIS on the server and for the life of me I cannot determine what version it is. Can anyone tell me how I can determine what version of IIS I have installed?