You can't, unless they enter it. Imagine being able to get that type of client information over any web page, can't be done.
You can setup security in most web servers to use NT authentication. In this case, the web server would pop up a login dialog and authenticate the user for you without extra code. Then once the user is authenticated, you could retrieve their username from the HttpServletRequest Object using the getRemoteUser() method. Check the Javadocs for more information. Wushutwist