Trojan horse programmes? Like the old Trojan Horse they are sneaky, harmful buggers that creep into your computer (posing, naturally, as a harmless executable file (*.exe) - a game, a screensaver etc.) and then "break out" once inside your computer and wreak merry havoc - usually by deleting files from your hard drive. They have to be activated, i.e. simply downloading the trojan won't kill your firstborn, you must actually run the programme. But when they are disguised as working-games etc., that's easily done. They are, technically, not viruses (they don't replicate) but they are just as damaging.