// Esta funcion devuelve la instancia del XMLHttpRequest - para AJAX
function createXMLHttpRequest( ) 
{
  try 
  { 
    return new ActiveXObject( "Msxml2.XMLHTTP"); 
  } 
  catch (e) 
  {    
  }
  
  try 
  { 
    return new ActiveXObject("Microsoft.XMLHTTP"); 
  } 
  catch (e) 
  {    
  }
  
  try 
  {
    return new XMLHttpRequest(); 
  } 
  catch(e) 
  {    
  }
  
  alert( "XMLHttpRequest not supported");
  
  return null;
}

function updateElement( elementId, url) 
{
  var xhr = createXMLHttpRequest();
  
  xhr.onreadystatechange = 
    function() 
    {
      if (xhr.readyState == 4) 
      { 
        // Request is finished
        if (xhr.status == 200)        
        {
          $(elementId).innerHTML = xhr.responseText; 
        } 
        else 
        {
          alert( "Error connecting to " + url + ", please try again later.");
        }
      }
    }
    
  xhr.open( "GET", url, true);
  xhr.send( null);		
}