SAN JOSE, Calif. – Hackers unleashed new variants of a computer worm that attacks a vulnerability in Microsoft Corp.’s Windows 2000 operating system, but infection rates appeared to be low and damage minor Wednesday.

The latest “War of the Worms” stands in contrast to previous outbreaks that brought networks and millions of PCs to a crawl in recent years.

It’s a sign, security experts say, that computer users are heeding warnings to quickly install patches as they’re released. It also indicates that Microsoft’s efforts to batten down the hatches of its ubiquitous software are paying off.

Several media outlets – including The New York Times, CNN and ABC – reported that the worms had invaded their networks. San Diego County was cleaning the bug from 12,000 computers. The worm blocked e-mails and slowed Internet connections in Massachusetts state government and caused delays at the Registry of Motor Vehicles.

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 antivirus research. He said the variations apparently had been programmed to compete with each other – one automated “bot” pushing the worm will remove another from an infected computer.

The number of potential victims was limited by the fact that only a vulnerability in Windows 2000 was remotely exploitable. The operating system was never marketed as a consumer product.

The damage was further reduced by businesses having become more aware of the risks of not having tight security.