Looking into the source code of the popup it comes from oz.valueclick.com.
Looking at the javascript within the pprune page you also have references to
//--></SCRIPT> <SCRIPT LANGUAGE=Javascript src="http://oz.valueclick.com/jsmaster"></SCRIPT>
<SCRIPT LANGUAGE=JavaScript><!--
if (ValueLoaded) ValueShowAd();
//--></SCRIPT> <NOSCRIPT>
So it looks as if it is being kicked off by pprune somewhere down the line
Martin