$(document).ready(function() {

  jQuery.fn.center = function() {

    this.css("position","absolute");

    this.css("top", ( $(window).height() - this.height() ) / 2+$(window).scrollTop() + "px");

    this.css("left", ( $(window).width() - this.width() ) / 2+$(window).scrollLeft() + "px");

    return this;

  }

  $("#id").center();

});