Security and anti-virus vendors are warning computer users of a new virus threat called Sasser.

Unlike most viruses that spread via infected e-mails, the Sasser worm spreads automatically, and can attack any computer connected to an Internet service provider.

It typically slows down the computer speed, shuts down the computer, automatically re-boots it and then cuts off Internet connection.

An anti-virus expert at Finnish Internet security company F-Secure said the virus is currently spreading rapidly throughout the world and may have already infected millions of computers.