jQuery.fn.center = function () {
    this.css("position","absolute");
    this.css("top", (($(window).height() - this.outerHeight()) / 2) + $(window).scrollTop() + "px");
    this.css("left", (($(window).width() - this.outerWidth()) / 2) + $(window).scrollLeft() + "px");
    return this;
}

function showAd(ad) {
    $("#"+ ad).dialog({
	   width: 650,
	   height: 675,
	   modal: true,
	   hide: "fade",
	   show: "fade"
    });
}