That would be the yahoo false positive...
http://forum.avast.com/index.php?topic=52692.msg446420#msg446420Hello guys,
this false positive will be corrected in new vps update in about 30 minutes. We are blocking ads from yieldmanager.com (by mistake because they were in our stats as distributor of Fake Antivirus websites).
Thank you for information you have provided.
Regards