function CreateXmlHttp()
   {
        //非IE浏览器创建XmlHttpRequest对象
    if(window.XmlHttpRequest)
    {
     xhr=new XmlHttpRequest();
    }

        //IE浏览器创建XmlHttpRequest对象   
 if(window.ActiveXObject)    
    {       try
            {
                 xhr=new ActiveXObject("Microsoft.XMLHTTP");    
             }
        catch(e)
            {
             try {
                  xmlhttp=new ActiveXObject("msxml2.XMLHTTP");
                 }
         catch(ex){}
            }
     }  
}