function InitAjax(){
  if(window.XMLHttpRequest) {
   ajax = new XMLHttpRequest();
   if (ajax.overrideMimeType) {
    ajax.overrideMimeType('text/xml');
   }
  }
  else if (window.ActiveXObject) {
   try {
    ajax = new ActiveXObject("Msxml2.XMLHTTP");
   } catch (e) {
    try {
     ajax = new ActiveXObject("Microsoft.XMLHTTP");
    } catch (e) {
    	ajax = false; 
    }
   }
  }
  if (!ajax) {
   window.alert("不能创建XMLHttpRequest对象实例.");
   return false;
  }
  return ajax;
 }