Many are claiming the Google result pages with YouTube results are triggering the security threat. The AVG warning reads:
Danger: AVG Search-Shield has detected active threats on this page and has blocked access for your protection.
Several searchers reported this but I doubt there is much Google can do to fix the situation. It seems like this would be on AVG's side.
Again, it seems to come up only when YouTube or other video results would show up in the search results.
Forum discussion at Google Web Search Help.