The Kelvir.B worm spreads using MSN Messenger when unwitting recipients click on a URL in a message reading: "lol! see it! u'll like it." Well (lol!) click on that and you will wind up downloading a variant of the Spybot worm, and sending a message to everyone else on your contact list.
Spybot is thought to be used by spammers. It includes software that gives third parties control of infected PCs, allowing them to be used as spam generators or in distributed denial of service attacks.
This is apparently the second virus that spreads using MSN Messenger; this is alarming considering that many organisations are planning to double their use of instant messaging in 2005, mostly by deploying private corporate networks.