We had this happen a few months back. Turns out it was a problem with our banner ad network (TribalFusion). They told us one of their IP addresses got on the virus list by mistake. They told us it would be fixed in the next update of the user's anti-virus software.
We said that was not acceptable. We suggested they swap that IP out. They offered no other solution and just kept telling us to re-install their code. We dropped them. It was making us look pretty bad, like we had a virus on our site.
Turns out we make more by running an Adsense image banner there than we did with their ads, so it was a net gain for us, and a big net loss for Tribal... ;-)
YES! I've been getting the same reports from my visitors since Friday 4/23. I have seen very little about this, so it appears Google isn't aware of the issue. So I have blocked all 3rd party networds on Adsense for now (102 of them!). I don't know yet if this will temporarily solve the issue.
I'd sure like to hear directly from Google on what they're doing about this, and an ETA when it will be fixed.
Every time I've seen this happen before it's turned out to be some weird thing with the AV program. I used to see it from time to time with Trend Micro. Then they'd issue another update, and it would go away.
I initially thought that too, unfortunately this is coming from visitors with different AV programs (Avast, Norton, McCafee, etc)...all at the same time. It's extremely unlikely all the AV programs have the same problem at the same time.
Not me (so far), but I am concerned about the effect this could have on the Adsense program. Have you reported it to Google? There is an Adwords feedback form at [adwords.google.com...] but it seems to be mainly for reporting offensive ads, not ads with malware.