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;
}