For those of you that are loading the site that Bob338 linked to ( hxxp://www.pcmech.com/forum/ ) are you actually loading various threads to test this? Because I go to that same site all the time and have the same problem, and the only time I get hit with the HTML: Iframe-inf detection is when I load each individual thread. I originally had to make an exclusion in Avast's Web Shield to be able to access the site at all, but now I just get the detection when I go into a forum and load a thread up.
On mine, Avast ends up alerting about this through the File shield by popping up the red box stating the offender as HTML: Iframe-inf, but the location it shows is my Temporary Internet Files folder on my hard drive. After visiting the site, I can do a scan of my drive with Avast and it will find these "threats" in that folder, with the names of the threads, like C:/Documents and Settings/*all the other stuff here*/Temporary Internet Files/Content.IE5/PTKCZ913/216397-norton-complains-about-attack-any-time-i-visit-pcmech-page[1].html. <That last part, from the numbers on, is the name of the thread where we're talking about the problem on that site, in case you're wondering. After scanning with Avast and deleting the problems, everything's fine until I go back there again. I've cleared out my Temp folders thinking maybe it was something left over from the old site (before the new site was launched recently, which is when the problems started) and have current definitions; 100824-1.