You apparently are unaware that there is a difference between a trojan horse and a virus.
A BIG difference. A “virus-prof” computer is no protection against a trojan, and you ought to know that.
A trojan targets the most vulnerable part of any system — the part between the chair and keyboard.
If a smooth-talking con man can talk a gullible user into defragging his hard drive with a ball-peen hammer, there is no computer software on the planet that can stop it.