More than likely an indication of a hacked web site, or infected .htm/l file on your system. The iframe tag in HTML is used to import data or run programs, so it entirely depends on what the payload of the site it is pointing at is as to what malware is involved.
Where is it found ?
Check the avast! Log Viewer (right click the avast 'a' icon), Warning section, this contains information on all avast detections. C:\Program Files\Alwil Software\Avast4\ashLogV.exe - Or check the source file using notepad C:\Program Files\Alwil Software\Avast4\DATA\log\Warning.log
####
When posting URLs to suspect sites, change the http to hXXp so the link isn't active (clickable) avoiding accidental exposure.