ajax问题

<!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>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<title>无标题文档</title>
<script language="javascript">
var xmlHttpRequest;
function createXMLHttpRequest()
{ if(window.ActiveXObject)
  {
                   return new ActiveXObject("Microsoft.XMLHTTP"); 
  }else
  {
      return new XMLHttpRequest();
}
}
function checkUserName(){ createXMLHttpRequest();

         xmlHttpRequest.open("GET","1.aspx",true);
         alert("dddd");
xmlHttpRequest.onreadystatechange=function()
{
  if(xmlHttpRequest.readyState == 4 && xmlHttpRequest.status==200)///status--%>
            alert("服务器返回"+xmlHttpRequest.responseText);
          }
xmlHttpRequest.send(null);
}
</script>
</head><body>
<input type="button" value="dddddd" onclick="checkUserName()"/>
</body>
</html>1.aspx页面
<%@ Page Language="C#" ContentType="text/html" ResponseEncoding="gb2312" %>
<%@ Import Namespace="System.Data" %>
<%
Response.Write("异步测试成功，很高兴!");
%>不能实现ajax