If you get it only on PPRuNe e-mail and nowhere else, and on three separate virus-protected computers, it sounds like a pop-up on the site. What's the URL of where you get it?
I've never seen it, but I'm using Firefox with the add-on pop-up blockers, and I don't see the ads on this site, either. I'll take a look with Firefox, and with the Internet Explorer I keep for checking stuff like this.