NTML retrieving using javascript

I would like to retrieve the username of the person logged onto windows 
for display on a logon screen in the mozilla browser.

I know that version <1.7 can now do ntlm authentification where mozilla 
sends this information back to the proxy, however i am having trouble 
finding where to access this information from within mozilla.

Has anyone done this before? or can someone point me in the right 
direction to find out how to access this information through javascript.

Thanks

Murray