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 + ",location=no");
      newwindow.focus();
    }