maximillianos - 11:54 am on Jun 9, 2010 (gmt 0)
We had this same problem recently. Narrowed it down to TribalFusion. When i contacted them they confirmed one of their ad server IPs was accidently flagged by AVG. They told me it was fixed with an update and to tell my visitors to update their AVG to fix the problem. Yeah right, like I'm going to try and tell a million visitors "hey if you avg please update it so you can use our site".
I told them why not just get a new IP for the ad server in question. They never responded. I removed their ad network about 3 months ago and relpaced it with Google image ads.
Funny thing is, G image ads out performed the old ad network. So we should have tested an replaced the ad network a long time ago.