var req = null;

if (window.XMLHttpRequest)
   req = new XMLHttpRequest ();
else if (window.ActiveXObject)
   req  = new ActiveXObject (Microsoft.XMLHTTP);

req.onreadystatechange = function() {
   if(req.readyState == 4 && req.status == 200)
      document.getElementById('footer').innerHTML = req.responseText;
}

req.open("GET", "footer.html", true);
req.send(null);