Symantec has affirmed that a buggy security update, which brought down millions of Chinese PCs, only affected computers with the Simplified Chinese version of Windows XP.

Users who applied a recent Norton update (KB924270) were faced with a blue screen of death upon reboot when the faulty signature quarantined critical system files netapi32.dll and isasrv.dll, mistaking them as Backdoor.Haxdoor Trojans.

Symantec spokeswoman Linda Smith Munyan said changes to the update automation, established to combat rising levels of encrypted malware, affected a definition which led to the false detection.

"[The change] inadvertently resulted in a change to a single definition used by the automated system and subsequently led to two files being falsely detected as malware."

While the update was pushed out of Symantec headquarters at about 1.00am Beijing time, the security company was alerted to the false positive around 9.30am and deployed a revised update four-and-a-half-hours later.

In the intervening thirteen-and-a-half-hour gap, China's state-sponsored Xinhau News Agency reported that the update had "caused millions of PCs and computers to crash, [which was] a heavy blow to people's daily work and ongoing business".

To restore their systems, users had to replace the missing .dll files, either from a Windows XP SP2 installation or recovery CD, or by copying the files from a working machine.

To help affected users, Symantec has offered the corrected update, a toll-free support telephone number, posted prominent warnings on its Chinese-language Web site, and has directed blue-screen-of-death casualties to restore their machines through Microsoft's Windows Recovery Console.

According to Munyan, the company's security response team "has made specific changes to the certification procedures that will help to prevent similar false positives, including adding additional checks before definitions are released".

The nearest event in scale to the Symantec false positive was a 2005 brouhaha after Trend Micro released a virus definition file that slowed thousands of PCs to a crawl and was estimated to have cost $US8.2 million.