I need to refresh a div which contains an ad while the rest of the page stays the same. I think I'm pretty close by reading the previous posts on this site, but one odd thing is tripping me up and I can't figure out why. When I call my function to refresh the div, it replaces everything on the page instead of just swapping out the div. I'd appreciate any help from you js experts!

Code:
<script type="text/javascript">
function adBump(rand){
//setup ad call. returns a document.write() command from ad server
var adcall = "http://oas.#*$!x.com/RealMedia/ads/adstream_jx.ads/www.#*$!x.com/#*$!x/"+rand+"other_paramters";

//adds the advertisement graphic above the ad when it loads.
document.getElementById("adLabel").innerHTML = "<img src='http://images.#*$!x.com/images/#*$!xx/adv_180x9.gif' width='180' height='9' alt='Advertisement'>";

//selects ad div, empties it then appends the new js call.
var b = document.getElementById("ad");
b.innerHTML = "";
var js = document.createElement("script");
js.type = "text/javascript";
js.src = adcall;
b.appendChild(js);

}
</script>

<div id="adArea">
<div id="adLabel">Ad label goes here</div>
<div id="ad">Ad goes here</div>
</div>