I too have had this on odd occasions when visiting the site. Not happening every time though. My eset blocks it and in the log I can see what it was. I have copied the log report to here , maybye it will help someone throw some light on the issue.
04/11/2011 06:16:52 HTTP filter archive viewtopic.php?t=14846 JS/Iframe.AP trojan connection terminated - quarantined Ronnie-PC\Ronnie Threat was detected upon access to web by the application: C:\Program Files\Mozilla Firefox\firefox.exe.
Log file says it is a JS/Iframe.AP trojan but again does not happen every time.
Hope this helps someone