function NewWindow(mypage, myname, w, h, t, l, scroll) {
    var winl = (screen.width - w) / 2;
    var wint = (screen.height - h) / 2;
    winprops = 'height='+h+',width='+w+',top='+wint+',left='+winl+',scrollbars='+scroll+',resizable'
    win = window.open(mypage, myname, winprops)
    if (parseInt(navigator.appVersion) >= 4) { win.window.focus(); }
}

// <a href="invoer.php" onclick="NewWindow(this.href,'Byelex','600','660','10','10','yes');return false;" target="_blank" class="directlink">Start de taxatie</a>