Vista - World's nastiest trojan fools AV software.

Pounces on banking passwords.
One of the world's nastiest password-stealing trojans evades detection by the majority PCs running anti-virus programs, according to a study that examined 10,000 machines.
Zeus, a stealthy piece of malware that sits on a PC and waits for users to log in to bank websites, is detected just 23 per cent of time by AV programs, according to the study (PDF) released by security firm Trusteer. Even AV programs with up-to-date malware signatures were unable to identify the infection a majority of the time, the authors said.