function popupwindow(myform, windowname, zheight, zwidth, zscrollbars)
{
    if (! window.focus)return true;
    var winl = (screen.width-zwidth)/2;
    var wint = (screen.height-zheight)/2;
    window.open('', windowname, 'height=' + zheight + ',width=' + zwidth + ',scrollbars=' + zscrollbars + ',top=' + wint + ',left=' + winl);
    myform.target=windowname;
    return true;
}