function addMap() {
  if (GBrowserIsCompatible()) {
    var map = new GMap2(document.getElementById("map"));
    var point = new GLatLng(51.5623,-0.0934);
    map.setCenter(point, 13);
    var marker = new GMarker(point);
    map.addOverlay(marker);
    GEvent.addListener(marker, 'click', addMessage);
  }
  function addMessage() {
    var message='This is where Chris lives';
    var msgNode=document.createTextNode(message);
    map.openInfoWindow(point,msgNode);
  }
}
window.onload=addMap;
window.onunload=GUnload;