Ads are slowing my site

[Jon12345]

I am struggling with this. Here is what I have done.

Firstly, I am using this code in place of where my advert was:

Code:

<!--CW replacement-->
<div style="width:300px; height:250px; background-color: #F2F2F2;">
<div id="load_top1"></div>
</div>

Then, right at the end of the footer after my Google Analytics tag, I am using this code:

Code:

<!--Load CW ad-->

<div id="load_top_footer" style="display: none;">
<script  src="http://tag.contextweb.com/TagPublish/getjs.aspx?action=VIEWAD&cwrun=200&cwadformat=300X250&cwpid=521268&cwwidth=300&cwheight=250&cwpnet=1&cwtagid=100009"></script>
</ div>
<script type="text/javascript">
window.onload = function () {
document.getElementById ('load_top1'). appendChild (document.getElementById ('load_top_footer'));
document.getElementById ('load_top_footer'). style.display ='';
}
</ script>

When I load my webpage, I can see the 300x250 placeholder there, but the advert never shows. Have I put my code in the wrong area or something?