JQuery :: Center An Element On Screen That Scrolls

I need to find out how to center an element on a form that scrolls. I have code (below) that displays a dialog, but I can't figure out how to make sure the dialog is centered on the page when the dialog displays as the user scrolls down the page. The dialog is displayed from a click on a href tag. I thought I could get the x,y coordinates where the href was click, but can't figure out how to center since the href can be at either the upper or low end of the page when it shown.

<script type="text/javascript">
var xCoord;
var yCoord;
$(document).ready(function () {
// if user clicked on button, the overlay layer or the dialogbox, close the dialog
$('a.btn-ok, #dialog-overlay, #dialog-box').click(function () {
[Code]...