A faulty update to Symantec's Norton Antivirus package has disabled "millions" of PCs in China, according to local press reports. 

One report carried by China's official news agency put the number of affected PCs in the millions, although others said that the figure was more like thousands or tens of thousands. The affected PCs cannot be started up. 

PCs running Windows XP began to fail after they downloaded a virus definitions update file on Friday. The regular updates are automatically pushed out from Symantec's servers.

Users explained that nothing went wrong immediately, but that the next restart showed the infamous Windows 'Blue Screen of Death' instead of the normal start-up sequence. The PCs could not be restored to operation by any normal means.

Symantec's China office explained in a statement that the software had mistakenly detected a virus in some key Windows XP system files. These files were either deleted or quarantined.

Users posting in online forums said that whole offices full of PCs had suffered the problem, which appears to afflict only PCs running the Simplified Chinese language version of Windows XP.

Symantec has sent out a revised update file, but this only fixes PCs that have not been restarted since applying the original faulty update.

The company has also described a command line procedure for replacing the deleted files with copies stored on the Windows XP installation CD-Rom.

Antivirus experts in China said that the issue is only affecting PCs with the Windows Update Service turned on, as the erroneously identified virus files were part of a security update applied to Windows late last year.