function createXMLHttpRequest() {
  if(window.XMLHttpRequest) {
    try { 
      xmlHttpRequest = new XMLHttpRequest();
    } catch(e) { return null; }
  } else if(window.ActiveXObject) {
    try {
      xmlHttpRequest = new ActiveXObject("Msxml2.XMLHTTP");
    } catch(e) {
      try {
        xmlHttpRequest = new ActiveXObject("Microsoft.XMLHTTP");
      } catch (e) { return null; }
    }
  } else return null;
  return xmlHttpRequest;
}