var Pool_xmlreq=function(){
var _cacheCase=[];
 _cacheCase.fetch=function(){
  var i=0;
  var _instance=function(){
   try{
    return (new ActiveXObject("Msxml2.XMLHTTP.5.0"));}
   catch(e){
    try{
     return (new XMLHttpRequest());}
    catch(e){
     try{
      return (new ActiveXObject("Msxml2.XMLHTTP"));}
     catch(e){
      try{
       return (new ActiveXObject("Microsoft.XMLHTTP"));}
      catch(e){
       alert("Error: initializing xmlhttprequest!");}}}}}
  for(;i<this.length;i++){
   if(this[i].readyState==0||this[i].readyState==4){
    return this[i];}}
  this[i]=_instance();
  return this[i];}
 this.length=function(){
  return _cacheCase.length}
 this.instance=function(){
  return _cacheCase.fetch()}
 this.connect=function(method,url,data,handler){
  var _xmlhttpreq=_cacheCase.fetch();
  with(_xmlhttpreq){
   open(method,url,true);
   setRequestHeader("Cache-Control","no-cache");  
   setRequestHeader("Content-Type","application/x-www-form-urlencoded;charset=UTF-8");
   send(data);
   onreadystatechange=function(){
    if(_xmlhttpreq.readyState<4)
     return false;
    if(_xmlhttpreq.status==200||_xmlhttpreq.status==304){
     handler(decodeURIComponent(_xmlhttpreq.responseText));
     return true;}
    alert("Error: status_"+_xmlhttpreq.status+"!");
    return false;}}}
}
var xmlreq=new Pool_xmlreq(); 

用法:
xmlreq.connect("post",url,postdata,function);