newwindow=false;
function fenster(url,breite,hoehe) {
  if(newwindow&&newwindow.closed==false)
    newwindow.close()
    var l = (screen.availWidth - breite) / 2;
    var t = (screen.availHeight - hoehe) / 2;
    newwindow = window.open(url,"newwindow","width=" + breite + ",height=" + hoehe + ",left=" + l + ",top=" + t + ";");
  newwindow.focus();
}