var peticion = false;
 var  testPasado = false;
 try 
 {
     peticion = new XMLHttpRequest();
 } catch (trymicrosoft) {
     try 
     {
       peticion = new ActiveXObject("Msxml2.XMLHTTP");
     } catch (othermicrosoft) {
       try 
       {
         peticion = new ActiveXObject("Microsoft.XMLHTTP");
       } catch (failed) {
         peticion = false;
       }
     }
 }
 if (!peticion)
     alert("ERROR AL INICIALIZAR!");