Security watchers have warned that the Kelvir.B worm has begun spreading around the world, dropping a payload in the form of another worm, known as Spybot, on infected PCs.

The worm spreads using MSN Messenger when unwitting recipients click on a URL in a message reading: "lol! see it! u'll like it"

Once clicked the link downloads a variant of the Spybot worm and sends a message to everyone else on the user's 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 the second virus that uses MSN Messenger to propagate and some fear it may have an impact on business adoption of instant messaging.

"Organisations are planning to double their use of instant messaging in 2005, mostly by deploying private corporate networks," said Graham Titterington, principal analyst at Ovum.

"There is confidence that instant messaging can bring substantial business benefits through speeding up decision making and improving communications.

"Users were worried about its intrusiveness, its disruptive capability, and its security implications. However these concerns diminish as users gain experience with the medium and become confident that they can effectively manage the risks."