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;
}