Here is what I can tell you from my experience...
Users get "Alerts" from antivirus programs.
Now there is what is called "FALSE POSTIVES" where there is something that is suspicious, but not really harmful. Yet the anti virus alerts anyway. Kind of a better safe then sorry kinda thing... You can read more about that
here. This happens ALOT. (BTW, AVG can be one of the worst)
Although rare, it can happen that adsense ads have a virus attached. However, Google is pretty good about weeding those out very quickly. In this case, there IS an adsense virus going around. SO it might be just that. Tell the guy at Moparts to Disable third party ads in his adsense account.
Viruses are a part of the internet. If you have a decent malware scanner and a decent virus scanner,
and both are up to date, your pretty safe. Its also a good idea to have the latest version of your browser, and tomake sure you have the lastest version of Flash and Java.
I recommend Norton 360 Antivirus and Malwarebytes. Its what I use, and I likely surf the internet more then any of you. I cant remember the last virus that did any damage to my system.
WARNING: If you dont want to be bothered paying for a decent anti virus scanner, then its your own fault when you catch a virus. Its like leaving your keys in the car and blaming the car manufacturer when it gets stolen.