Avast? Never heard of it.
I'm using Avast and I think it's excellent (and free!).
After experiencing this for the first time this morning, I asked my guild's webmaster about it. He checked out the site and stated that the problem is an ad from tubemogul.com which is misaligned.
No, I don't know exactly what that means, but he has some considerable experience with this sort of thing and I trust his judgement. I thought I would pass the word along, perhaps it will help resolve the issue.