newwindow=false;
function fenster(url, hoehe, breite) {
  if(newwindow&&newwindow.closed==false)
    newwindow.close()
    var l = ((screen.availWidth - breite) / 1.5);
    var t = ((screen.availHeight - hoehe) / 2)*0.5;
    
    newwindow = window.open(url,"newwindow","scrollbars=yes, width=" + breite +
                                            ",height=" + hoehe +
                                            ",left=" + l +
                                            ",top=" + t + ";");
  newwindow.focus();
}