function centerWindow(p_url, p_title, p_width, p_height, p_extra) {
    var xMax;
    var yMax;
    if (document.all) {
        xMax = screen.width;
        yMax = screen.height;
    } else {
        if (document.layers) {
            xMax = window.outerWidth;
            yMax = window.outerHeight;
        } else {
            xMax = 800;
            yMax = 600;
        }
    }
    var xOffset = (xMax - p_width) / 2, yOffset = (yMax - p_height) / 2;
    p_extra = (p_extra == null) ? '' : (',' + p_extra);
    return window.open(p_url, p_title, 'width=' + p_width + ',height=' + p_height + ',screenX=' + xOffset + ',screenY=' + yOffset + ',top=' + yOffset + ',left=' + xOffset + p_extra);
}