When nmap can't ID the OS...

[Derek Fountain <dflists () iinet net au>]

I have a remote server, which according to Netcraft, is running IIS-5.0 on 
Windows2000. I don't know for sure if that's the case, but it is believable.

When I used nmap (3.75) to ID the OS on it, it says it doesn't know. The 
machine has several ports open and none filtered, so I'd have thought nmap 
could work it out. I have a local machine running W2K and nmap can ID that 
(at least as far as telling me it's running some version on Windows).

What could be up with the remote machine that stops nmap IDing the OS it is 
running?