跪求高手解答一下关于Ajax的一个问题。

通过Button的onclick="DoAjax('Ajax.aspx', CallBack)"调用时，总是提示缺少对象？怎么回事儿？
再有就是Javascript怎么向服务器提交xml格式文件呢？我用XMLHttpRequest.send(xml);发送时，好像用为xml
包含了特殊符号<,/,>无法发送一样，应该怎样处理呢？JavaScript用没用想asp.net中htmlEncode编码方法啊？

<script type="text/javascript">
var xHRequest;

function CreateXHR(){
  if (window.ActiveXObject){
  xHRequest = new ActiveXObject("Msxml2.XMLHTTP.3.0");
  }
  else if (window.XMLHttpRequest){
  xHRequest = new XMLHttpRequest();
  }
  else alert("Your browser don't support XmlHttpRequest!");
}

function DoAjax(RequestURL, Fun){
  CreateXHR();
  if (xHRequest){
  xHRequest.onreadystatechange = Fun;
  xHRequest.open("post", RequestURL, true);
  xHRequest.setRequestHeader("Content-Type","application/x-www-form-urlencoded;charset=UTF-8");
  // xHRequest.send("&lt;fjdaslfj&gt;");
  xHRequest.send(CreateXML());//为什么这里总是Microsoft JScript 运行时错误: 缺少对象呢？
  }
}

function CallBack(){
  if (xHRequest.readyState == 4){
  if (xHRequest.status == 200){
  var a =