Experts said there were 11 variations of the worm, which first emerged at the weekend. The Finnish computer virus experts F-Secure Corp said the creators had programmed them to compete with each other. It said US-based systems appeared to be the worst affected .
The worms had been causing most problems at companies with large, networked computer systems, rather than among individual computer users, said David Perry, a security analyst at Trend Micro Inc, a computer security company. He said the worms could attack a system without needing to open any software, so some users would have been infected without knowing it.
A common symptom of the attack is computers repeatedly restarting. Experts say installing the Microsoft patches is the best preventative action.