function creaAjax(){
  var objetoAjax=false;
  try {
   /*Para navegadores distintos a internet explorer*/
   objetoAjax = new ActiveXObject("Msxml2.XMLHTTP");
  } catch (e) {
   try {
     /*Para explorer*/
     objetoAjax = new ActiveXObject("Microsoft.XMLHTTP");
     } 
     catch (E) {
     objetoAjax = false;
   }
  }

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

function ejecutar(url,capa)
{
   	var ajax=creaAjax();
   	var capaContenedora = document.getElementById(capa);
	ajax.open("GET", url, true);
    ajax.onreadystatechange = function(){		
         if (ajax.readyState==1) {
                 capaContenedora.innerHTML="<img src='images/loading.gif'/>"
         }else if (ajax.readyState==4){
            if(ajax.status==200){ 
                 capaContenedora.innerHTML=ajax.responseText; 
            }else if(ajax.status==404){
				 capaContenedora.innerHTML = "La direccion no existe";
            }else{
                 capaContenedora.innerHTML = "Error: ".ajax.status;
            }
        }
	}
   	ajax.setRequestHeader('Content-Type','application/x-www-form-urlencoded');
   	ajax.send(null);
   	return
}