Hi Jane,
so let's cover some bases.
a) the "Browser" running on the client machine is running JavaScript, but it "isolates" that JavaScript from the machine itself. So it's not possible in JavaScript to get things like the Windows Login etc.
b) Bear in mind that you can run a browser on anything, so on many platforms (like say an iPhone) the windows login does not apply anyway.
c) If JavaScript is limited, then you need something else to "bridge" the gap between browser and machine. In the case of IE this is an ActiveX control (and example 45 - wScriptActive shows you how this is done.) but of course this technique only works in IE, other browsers would require other techniques. Actually, it won't even necessarily work in IE, if the user has support for ActiveX turned off it'll fail there too.
d) I've done a search, but I don't see how it's possible in chrome. So any more information you have on that would be useful.
Cheers
Bruce