Share your stories of the most destructive, dangerous, annoying, or comedic viruses/Trojans/spyware/worms that you have seen!
My story takes me back to high school. My friend had found this (supposedly) non-destructive Trojan on a forum board, and decided to load it on his PC in the classroom. The computers were running a program that only allowed you to work on a fake image of the OS, and after a restart, would erase all changes made to that template; so in short he didn't care what happened.

Anyway, this little bugger would spawn multiple IE windows that bounced happily all over the screen. To compliment that, each window contained a flashing smiley face, and proudly portrayed "YOU ARE AN IDIOT!" To further add insult to injury, a lovely chant played in the background proclaiming "YOU ARE AN IDIOT! HA HA-HA-HA-HA-HA-HA-HA HA-HA-HA-HA-HA!" If you attempted to close one of the IE windows, it would respawn itself and possibly more merry copies! As a final straw, a pop-up window would periodically show up to display, you guessed it, "YOU ARE AN IDIOT!"
So annoying...