protected void Page_Load(object sender, EventArgs e)
    {             
        HttpCookie cookie = Request.Cookies["hello"];

        if ((cookie != null))
        {
            Label1.Text = cookie.Value;
            TextBox1.Text = cookie.Value;
            //Response.ContentType = "text/HTML";
            Response.Write(cookie.Value);
            Response.End();
        }
        else
        {
            Response.Write("no Cookie");
            Response.End();
        }
    }

    <script language="JavaScript"> 
           var xmlHttp ; 
        function createXMLHttpRequest() 
        { 
            if(window.ActiveXObject) 
            { 
                xmlHttp = new ActiveXObject("Microsoft.XMLHTTP"); 
            } 
            else if(window.XMLHttpRequest) { 
                xmlHttp = new XMLHttpRequest(); 
            } 
        } 
               function doPostBack() 
        { 
          var url = "http://www.aaa.com/test1.aspx"; 
      
            createXMLHttpRequest() ; 
            xmlHttp.open("GET",url,true);                        
            xmlHttp.onReadyStatechange = showCallBack;               
            xmlHttp.send(null); 
        } 
        //回调函数 
        function showCallBack() 
        { 
            if(xmlHttp.readyState==4) 
            {        
                if(xmlHttp.status==200) { 
                   success(); 
                } 
            } 
        } 
        //解析xml 
        function  success() 
        { 
            var responseXml; 
            responseXml = xmlHttp.responseXML;          
            var text= xmlHttp.responseText; 
                if(text!=null&&text.length>0) 
            { 
               alert(text);
            } 
            else 
            { 
                info = xmlHttp.responseText; 
                alert(info); 
            } 
        }     </script>