Grisoft's newest version 8.0 of AVG anti-virus is messing with my logs. AVG Search-Shield is automatically enabled and follows all links in search engine results to see if they are 'safe'. If I have 2 links in a search result (organic & Adwords) it follows both even though the user never comes to my site. In my logs I see 2 hits on each of the 2 links. It disguises itself as Internet Explorer (fakes the UserAgent) to get around any blocks and shows no referers in my logs. I can't tell what is real traffic and what is AVG. From what I can tell I am not being charged by Adwords for these AVG follows but I have to investigate further. User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1)
Apparently AVG does not click Adwords. http://www.theregister.co.uk/2008/06/19/avg_linkscanner_and_adwords/
There have been some issues with AVG sending out fake traffic recently. Maybe you are noticing some of those effects?