computer questions

[GateandGarden]

I currently have a trojan horse on my computer. I'm not at home; I'm at a computer somewhere else. My computer at home is off because I've been told that the virus can't do anything if I have the computer turned off. There's so much I don't understand about this and I was thinking some or many of you might have some or all of the answers.

First of all, I don't understand how I got this thing. I was told by the people from whom I bought the computer that I must've downloaded it from somewhere. Recently I've downloaded Quintessential CD player, but I thought that program was legit. (?) Of course I've also downloaded from various FM sites that I've gotten access to from throwndown.net. I didn't think there was anything wrong with that. What are the chances that one of those fan sites would have a trojan horse?

Why didn't Norton Antivirus stop it from infecting my computer? I'm kind of wondering what the purpose is of paying 60 dollars for Norton Antivirus if this still happens.

What is the purpose of a trojan horse? What is the person who put on my computer getting out of it? Can this cause identity theft? Is someone looking at my photos of Linda Blair naked?