function createXmlHttp() {
  var xmlHttp;
  try{
    if( window.ActiveXObject ) {
      for( var i = 5; i > 0; i--) {
        try{
          if( i == 2 ) {
            xmlHttp = new ActiveXObject('Microsoft.XMLHTTP');
          }else{
            //此处不知为何用4.0、5.0会出现乱码
            //xmlHttp = new ActiveXObject( "Msxml2.XMLHTTP." + i + ".0" );
            xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");
          }
          //这些方法为何不能用？？？
          //xmlHttp.setRequestHeader("Content-Type","text/xml");
          //xmlHttp.setRequestHeader("Content-Type","GBK");

          break;
        }
        catch(e){
           xmlHttp = false;
        }
      }
    }else if( window.XMLHttpRequest ){
      xmlHttp = new XMLHttpRequest();
      if (xmlHttp.overrideMimeType) {
        xmlHttp.overrideMimeType('text/xml');
      }
    }
  }catch(e){
    xmlHttp = false;
  }
  return xmlHttp;
}