To read it from the client is difficult with standard security settings but this would work on low settings:
Code:
var oNet = new ActiveXObject("WScript.Network");
alert(oNet.UserName);
If you are using ASP and the client is authenticated you can use code on the server:
Code:
var sUserName = Request.ServerVariables("AUTH_USER");
There are similar methods using PHP, JSP etc.
--
Joe