Software sold in China by Symantec, the world’s largest computer security company, Friday suddenly attacked the very PCs it was supposed to protect.

The surprise switch by Symantec’s Norton anti-virus software from digital gamekeeper to poacher hit users of Chinese-language versions of Microsoft’s Windows XP operating system.

An online update of the anti-virus software prompted it to confuse inoffensive but essential XP files as a hostile “Trojan horse” and to delete them. Affected PCs that are switched off cannot be restarted unless the files are reinstalled.

Symantec declined to say how many users had been affected, but local rival security software supplier Rising Tech said the problem had prompted more than 7,000 individuals and hundreds of companies to clog its customer service lines.

“Norton AntiVirus causes operating systems to collapse, millions of computers face total disaster,” Rising Tech said on its website.

“Anti-virus experts say this is the biggest case of an erroneous computer virus report in the country for nearly five years,” said Kingsoft, another Chinese computer security software vendor.

Symantec declined to comment on why it had not published information about the error on its website, but said it had issued a corrected update to the anti-virus software by early Friday afternoon, Chinese time.

The mishap is likely to add to the challenge facing the US company as it seeks to expand in the highly competitive and fragmented Chinese internet security market.