function Ajax2(){ 
  var xmlhttp=false; 
  try { 
   // Creación del objeto ajax para navegadores diferentes a Explorer 
   xmlhttp = new ActiveXObject("Msxml2.XMLHTTP"); 
  } catch (e) { 
   // o bien 
   try { 
     // Creación del objet ajax para Explorer 
     xmlhttp = new ActiveXObject("Microsoft.XMLHTTP"); } catch (E) { 
     xmlhttp = false; 
   } 
  } 

  if (!xmlhttp && typeof XMLHttpRequest!='undefined') { 
   xmlhttp = new XMLHttpRequest(); 
  } 
  return xmlhttp; 
} 


function Ajax() { 

   var req; 

   if(window.XMLHttpRequest){ 
      // Firefox, Safari, Opera... 
      req = new XMLHttpRequest(); 
   } else if(window.ActiveXObject) { 
      // Internet Explorer 5+ 
      req = new ActiveXObject("Microsoft.XMLHTTP"); 
   } else { 
      // There is an error creating the object, 
      // just as an old browser is being used. 
      alert('Problem creating the XMLHttpRequest object'); 
   } 

   return req; 

}