if (window.XMLHttpRequest) { // Usado para Mozilla, Safari  
     http_request = new XMLHttpRequest();  
   } else if (window.ActiveXObject) { // Usado pro IE  
       try {  
           http_request = new ActiveXObject("Msxml2.XMLHTTP");  
     } catch (e) {  
         try {  
             http_request = new ActiveXObject("Microsoft.XMLHTTP");  
         } catch (e) {}  
    }  
 }