sometimes warns me my pc has been attacked by Trojan horse, that gives me an impression that Trojan horse could be planted via TCP/UDP/HTTP and other legitimate ways. I am kind of confused by their warning message then. You name email is one of the legitimate ways to plant a Trojan, other networking activities such as UDP/TCP access could also be used as a way to sneak Trojan horses to the pc?

I appreciate your time! Thanks.