put this at the top of your page (obviously changing things like width/height e.t.c..)
<SCRIPT LANGUAGE="JavaScript">
<!-- Begin
function popUp(URL) {
day = new Date();
id = day.getTime();
eval("page" + id + " = window.open(URL, '" + id + "', 'toolbar=0,scrollbars=0,location=0,statusbar=0,menubar=0,resizable=1,width=width,height=height,left = 312,top = 234');");
}
// End -->
</script>
then where you want the new page to pop up just do...
<script language="javascript">popUp("http://www.yoursite.com/errorpage")</script>
and I think it should work :p