If you are connected to the internet, you can contract AIDs computer viruses. They might even be sent directly to you. Now, I'm assuming no one is out to get you and maliciously sent you the virus. Also, why would adds give viruses? They wouldn't be effective adds then. Imagine seeing an add to buy something, and you are toying with thoughts of buying it. Then you find out that add sent you a virus. Would you still want to buy the item? I wouldn't Also, you said that a message came up saying it was detected. It could have been a delayed message. Maybe you got the virus, it started working it's way in, and several minutes later, it was finally found. Then a message pops up saying it was found.
It could really be any number of things.