function popup(url, windowname, width, height) 
{
    var left   = (screen.width  - width)/2;
    var top    = (screen.height - height)/2;
    var params = 'width='+width+', height='+height;
    params += ', top='+top+', left='+left;
    params += ', directories=no';
    params += ', location=no';
    params += ', menubar=no';
    params += ', resizable=no';
    params += ', scrollbars=no';
    params += ', status=no';
    params += ', toolbar=no';
    var newwin=window.open(url, windowname, params);
    if (window.focus) { newwin.focus(); }
    return false;
}