I recently experienced this one when somebody asked for help with an issue they were having with their machine. It was a browser issue (IE10 vs. FireFox) but they insisted it was a virus. No, no... I demonstrated what the issue was & tried explaining about IE's "compatibility mode" and other settings. No matter what I said they were convinced it was a virus. They thanked me for my time and said they'll take it to Best Buy to have their "Geek Squad" remove the virus.