创建XMLHttpRequest

function createXMLHttpRequest(){
 if(window.XMLHttpRequest){
  XMLHttpReq = new XMLHttpRequest();
  return XMLHttpReq;
 }else if(window.ActiveXObject){
  try{
   XMLHttpReq = new ActiveXObject("Msxml2.XMLHTTP");
   return XMLHttpReq;
  }catch(e){
   try{
    XMLHttpReq = new ActiveXObject("Microsoft.XMLHTTP");
    return XMLHttpReq;
   }catch(e){}
  }
 }else{
  alert("no XMLHttpReq has create");
 }
}