// IE için
var objUserInfo = new ActiveXObject("WScript.network");
document.write(objUserInfo.ComputerName+"<br>"); 
document.write(objUserInfo.UserDomain+"<br>"); 
document.write(objUserInfo.UserName+"<br>");  
var uname =  objUserInfo.UserName;

// Firefox ve Chrome için
return Components.classes["@mozilla.org/process/environment;1"].getService(Components.interfaces.nsIEnvironment).get('USERNAME');
}