can't undersatand where does the pop up come from

[eluser]belbek[/eluser]
I have a simple page in a site built on CI. In that page a just have flv player using SWF-object.
While playing the flv long pop up appears on top of the page with advertising and link leading to some other site. There is nothing strange in the source code. Is this XSS. If it is how to treat it. This behaviour appears only in IE 6,7. $config['global_xss_filtering'] set to true in the congig file.
Thanks.