I liked this one because its a clever new alternative on a virus. Not that I approve of viruses at all, but originality has to be recognised!
Basically, the virus (not sure why they call it a Trojan, possibly because it sneaks in by a back door in, unsuprisingly, Internet Explorer) encodes a load of files using PGP (pretty good privacy) ecryption. It is normally used for email signatures and things, and needs a key to decrypt it. Normally you know the decrypting key, but in this case you'd have to guess the key (very difficult considering you don't know the person and couldn't guess what they might have used) or brute-force attack it. Since an average brute force attack takes 3 billion MIPS years (three million million years of a 1 Million Instructions Per Second computer running on it all the time) then you're going to be waiting a while (even with modern computers that run at thousands of MIPS)
http://news.bbc.co.uk/1/hi/technology/4580389.stm