The Sasser worm can infect any computer that is switched on and contrary to most other worms or viruses is not spread by email, said Mikko Hyppoenen, head of anti-virus research at the Finnish internet security firm F-Secure.

"This is one of few worms that spreads automatically. It is enough for your PC to be on," Hyppoenen said.

The worm typically shuts down the computer, then automatically re-boots it, repeating the procedure several times.

A type of computer virus, worms are self-contained and are able to spread without hitching a ride on something else and without human intervention (such as opening an email).

Harmless

Hyppoenen said the worm, while inconvenient, was harmless.

"This worm does not have any criminal intentions, unlike the Bagle and Sobig viruses we saw earlier which took control of computers by opening back doors to send spam. Sasser doesn't do anything," he said.

The anti-virus expert said it was not yet known who was behind the latest attack, but suspected it was teenage hackers out to have some fun.

"It was probably some hobbyist, a teenager who has the skills and wants to show off," he said.

Sasser was first observed at 00:01 GMT on Saturday, and was infecting computers that had not installed the latest Microsoft patch in the past 18 days.

Affected computer users can solve the problem by visiting an internet security site or Microsoft and downloading the required patch.