Looks like you are posting out of Canada. There is a possibility you are seeing advertisments that many of our other users would not be, which means that this will be somewhat more difficult to track down.
We need to know exactly what information was displayed in that antivirus message. There should be a log file somewhere that would indicate any possible virus variant, where it came from, what IP address it was logged from, etc.
There is a good possibility that this is not related to our site. It may have just been some bot fishing for IP ranges that triggered your firewall at a coincidental time.
I will garuntee 100% that none of the site code on our servers is compromised in any way. We have very robust antivirus processes in play on the servers, and if there was even a slight possibility of bad code, we would have dozens of people in here reporting a problem.
Please read the thread in here entitled "how to report bad ads" and follow those steps if you recieve this message again.
We take all reports of virus activity very seriously, as our reputation is at stake if nothing else, so please provide any information you can give us to help us determine the root cause of the issue.