You can not do that....
What
you should do is the following, use layers with an iframe to create what looks to be the effect you want.
This way you would have full control over it and will not have to worry about pop up blockers...
<div id="alert_div">
<iframe></iframe>
</div>
and you show and hide the div
document.getElementById("alert_div").style.display="none"; //hide
document.getElementById("alert_div").style.display="block"; //visible
search this forum or the web on how to use iframes.
Eric