AJAX提交后Response.Write（）和ClientScript.RegisterStartupScript没反应

大家帮我看下问题出在哪，为什么后台Page_Load中　　
Response.Write("<tr><td>是</td></tr>");
和
ClientScript.RegisterStartupScript(this.GetType(), "Startup","<script>alert('是!')</script>"); 
执行后前台都没有反应啊，这些语句都单独调试过没问题的，以下是代码：<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="WebApplication1._Default" %><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title>无标题页</title>
    <script type="text/javascript">
    
    //获取XMLHttpRequest
    function getXMLHttpRequest()
    {
        var xmlHttp;
        try
        {
            // Firefox, Opera 8.0+, Safari
            xmlHttp=new XMLHTTPRequest();
        }
        catch (e)
        {
            // Internet Explorer
            try
            {
                xmlHttp=new ActiveXObject("Msxml2.XMLHTTP");
            }
            catch (e)
            {
                try
                {
                    xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
                }
                catch (e)
                {
                    return false;
                }
            }
        }
        return xmlHttp;
    }
    
    //获取XMLHttpRequest并提交
    function callServer(msg)
    {
        if (msg=="")
        {
            return;
        }
        var xmlHttp;
        xmlHttp = getXMLHttpRequest();
        if (!xmlHttp)
        {
            alert("您的浏览器不支持AJAX！");
            return;
        }
        var str="Default.aspx?msg="+msg;
        xmlHttp.open("GET",str,true);
        xmlHttp.send(null);
    }    </script>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <input id="Text1" type="text"  onkeyup="callServer(this.value)" />
        <input id="Text2" type="text" onkeyup="callServer(this.value)" />
        <div id="main" style="width: 314px; height: 271px">
        </div>
    
    </div>
    </form>
</body>
</html>
protected void Page_Load(object sender, EventArgs e)
        {
            if (Request.QueryString["msg"] != null)
            {
                string strMsg = Request.QueryString["msg"].ToString();
                switch (strMsg)
                {
                    case "yes":
                        Response.Write("<tr><td>是</td></tr>");
                        if (!ClientScript.IsStartupScriptRegistered("Startup"))
                            ClientScript.RegisterStartupScript(this.GetType(), "Startup","<script>alert('是!')</script>"); 
                        break;
                    case "no":
                        Response.Write("<tr><td>否</td></tr>");
                        if (!ClientScript.IsStartupScriptRegistered("Startup"))
                            ClientScript.RegisterStartupScript(this.GetType(), "Startup", "<script>alert('否!')</script>"); 
                        break;
                    default:
                        Response.Write("<tr><td>无</td></tr>");
                        if (!ClientScript.IsStartupScriptRegistered("Startup"))
                            ClientScript.RegisterStartupScript(this.GetType(), "Startup", "<script>alert('无!')</script>"); 
                        break;
                }
            }
        }