function popup(url, name, width, height)
{
  if (screen) {
    w = screen.availWidth;
    h = screen.availHeight;
    if (width > w) width = w;
    if (height > h) height = h;
    leftPos = (w / 2) - width/2;
    topPos = (h / 2) - height/2;
 }
  settings =
    "toolbar=no,location=no,directories=no,"+
    "status=no,menubar=no,scrollbars=yes,"+
    "resizable=yes,width="+width+",height="+height+
    ",left="+leftPos+",top="+topPos;
  window.open(url,name,settings);
  return false;
}