We'll need more information than that to investigate further. Do you have the log of the threat from your Antivirus? it should show what IP addresses the attack vector came from, what threat it was, exact date and time, etc. With that information we should be better able to determine if it came from us or one of our ads, or was just coincidental. Usually if there is any type of even suspect ad, we will get multiple reports, but I'll keep an eye out.
Some antivirus programs also record some ad related cookies as intrusions, so there is a chance it was something along those lines served from an advertiser. Or something coincidental in timing but not necessarily related to our site.
Please get the log information to us as soon as possible.