IIS

Try this:
go to
c:\winnt\system32\inetsrv\inetinfo.exe

Right click this file and click the version tab

IIS4 is the version of IIS your are most likely running. It installs from the NT4 Option Pack.
If you want to check this remotely, there is a fun way. Open a command prompt and type:
TELNET WebServerIPAddress 80

Replace "WebServerIPAddress" with the IP address of the web server (makes sense!). Once the white Telnet screen opens, start pressing the Enter key, Space key, and letter keys on the keyboard. YOu want to send an invalid character to the web server, so it will disconnect the connection. When the Telnet session is aborted, you will have text on the screen, part of which is the beginning of the HTML.
But the very first few lines on text will say something like:

HTTP/1.1 400 Bad Request
Server: Microsoft-IIS/4.0
etc...

Now you know the web server type that you conneted to. This works for all web servers, from what I have read.

Hope this helps.