The worm exploits a security flaw, but this can be prevented with a Microsoft patch.
It typically shuts down the computer then automatically re-boots it and repeats this process several times, but is not thought to cause lasting damage.
Mikko Hyppoenen, an anti-virus expert at Finnish internet security company F-Secure, said "It was probably some hobbyist, a teenager who has the skills and wants to show off. We expect things to get much worse when people bring their laptops in to the office after the weekend."
Graham Cluley, senior technology consultant for Oxfordshire-base software security company Sophos, said home users were especially vulnerable.
"They are often not running the latest anti-virus protection, haven't downloaded the latest security patches from Microsoft, and may not be running a personal firewall," he added.