js无刷新入门问题

Default.aspx.cs代码
public string xx = string.Empty;
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            string name = this.Request.QueryString["name"];
            if (name != null)
            {
                Response.Write(name);
                Response.End();
            }
        }
    }
Default.aspx代码<!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>user Validation</title>
    <script type="text/javascript">
        var xmlhttp;
        function Validation() {
            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) {
                        alert("您的浏览器不支持AJAX！");
                        return false;
                    }
                }
            }            var name = document.getElementById("Text1");
            xmlhttp.open("Post","Default.aspx?name="+name.value,true);
            xmlhttp.onreadystatechange = function () {
                if(xmlhttp.readyState == 4) {
//                    alert(xmlhttp.responseText);
                    document.write(xmlhttp.responseText);
                }
            }
            xmlhttp.send(null);
        }
    </script>
    
</head>
<body>
    <form id="form1" runat="server">
    <div>
    
    </div>
    aaaa
    <input id="Text1" type="text" />
    <input id="Button1" type="button" value="button"  onclick="Validation()"/>
    <%=xx %>
    </form>
    
</body>
</html>
问题是，我按了按妞。xx的内容出来了，可原来的内容没拉。
另外，哪个大大给我介绍个可以学c#.net ajax的网站，多点例子，谢谢。。