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();
}