How do I detect Windows NT with javascript?

Well... not necessarily. I was in fact noticing the other day that my Netscape 4.03 on WinNT did in fact identify itself as NT in the user agent string. I am not on that machine right now or I'd put the value here, I will later if noone else does in the interim.

It is correct that you can't be 100% sure what platform/browser the user is on.  But when you use javascript you can never be anywhere close to 100% sure the code will execute properly or at all, so for many purposes it's ok.

A valid point - Communicator is the first version of Netscape (or any other browser I'm aware of) that does, in fact, have an NT-specific version - but it's entirely possible to run the W96 version on an NT box or vice versa.

Another note from "in the meantime" - I've been goofing around a bit at home, running Windows in emulation mode on a Mac, and the appVersion identified me as a WindowsNT user.  What fun!  I suppose the same thing would happen on a UNIX terminal emulating windows, etc.

All of the following are on NT4 workstation.

Netscape Navigator 4:
4.03 [en] (WinNT; U ;Nav)

Internet Explorer 3.02:
2.0 (compatible; MSIE 3.02; Windows NT)

Netscape Navigator 3:
3.0Gold (WinNT; I)

The browser seems to have some sense typically as to what platform it's running on, but as indicated before can't be trusted completely. So to answer the original question: if absolute reliability isn't important, it seems you should use the following:

else if (navigator.appVersion.indexOf("WinNT") != -1
      || navigator.appVersion.indexOf("Windows NT") != -1)
 document.write('<font size="3"><b>Windows NT</b></font>');

What are you going to do with this?