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();
}