Well, the guy was infected with the trojan and according to what I heard, all of the porn files were dated a few days after infection occured, so it could be a valid argument. Most people that collect porn do so for years. It would be pretty bad timing to have just started collecting and then get caught for it.
Like Zone says, they have to prove it beyond any doubt.
When this thread read "Trojan man" I thought it was meant the guy from all the Trojan condom commercials. :P
