Those IM programs, like Messenger, Yahoo IM etc are an absolute security nightmare; it's like installing a cat flap in your firewall, things can get in and out that easily. I know of at least one major company who had absolutely airtight IT security policies in every other respect, but quite happily let their employees use these for communication (ostensibly for use only with global partners, like anybody cared) - the subsequent clean up, after the Trojans, was apparently quite costly!
