我用JavaScript实现ajax，为什么不成功啊

我在webform2.aspx用ajax向webform1发送请求，目前情况是这样的，用vs调试时 ，用户名和密码能跳到webform1里，；
用firebug在火狐里面是执行调试时，也能检测到req.readState == 4 req.status == 200啊，但就是为什么document.getElementById("display").innerHTML = req.responseText;
alert("你好，ajax");没执行呢？大家帮忙看看呗。

<script type="text/javascript">

        var req;

     function creatReq()

        {

            var userName = document.getElementById("txtUser").value;

            var pwd = document.getElementById("txtPWD").value;

            var url = "WebForm1.aspx?userName=" + userName + "&pwd=" + pwd;

            req = new XMLHttpRequest();

            if (req) {

                req.open("GET", url, true);

                req.onreadystatechange = function () {

                    if (req.readState == 4 && req.status == 200) {



                        // if (((String)(req.responseText)).substr(0, 4) == true)

                        document.getElementById("display").innerHTML = req.responseText;

                        alert("你好，ajax");



                    }





                }

               

              

                req.send(null);            

            }



        }

       

        

       

        function judge()

        { creatReq(); }

 <input id="Hidden1" type="hidden" />

        <span id="Label1">用户名</span>

        :<input name="txtUser" type="text" id="txtUser" />

        <br />

        <span id="Label2">密码：</span>

        <input name="txtPWD" type="text" id="txtPWD" />

        <br />

        <input id="btnLogin" type="button" value="登录" onclick="judge()" />

     <span id="display">11111</span>

服务webform1.aspx代码：

using System;

using System.Collections.Generic;

using System.Web;

using System.Web.UI;

using System.Web.UI.WebControls;

using System.Data;

 

public partial class WebForm1  : System.Web.UI.Page

{

    protected void Page_Load(object sender, EventArgs e)

    {

        string nam = Request.QueryString["txtUser"].ToString();

    

 string pwd = Request.QueryString["txtPWD"].ToString();

       

        if (nam=="yalunwang"&&pwd=="123")

        {

            Response.Write("HELLO,AJAX");

        

 

    }

}