San Francisco ? Several new variants of a computer worm emerged Wednesday to attack corporate networks running the Windows 2000 operating system, just a week after Microsoft Corp. warned of the security flaw.

As experts predicted, the Windows hole proved a tempting target for rogue programmers, who quickly developed more effective variants on a worm that surfaced over the weekend and by Tuesday had snarled computers at several large companies.

Among companies affected by the worm and its variations were ABC, CNN, The Associated Press, The New York Times and Caterpillar Inc. In California, San Diego County said it needed to cleanse 12,000 computers of the bug. ABC News producers had to use electric typewriters Tuesday to prepare copy for their “World News Tonight” broadcast, according to spokesman Jeffrey Schneider.

On Wednesday, four new variants of the worm had been detected by F-Secure Corp. in Finland, bringing the total to 11, said Mikko Hypponen, the company’s manager of anti-virus research.