Several outlets, including the BBC, reported that harmful software was being pre-installed on PCs at the manufacturing stage.

Microsoft's Digital Crimes Unit has said that malware was not being installed in factories, contrary to earlier media reports.

A spokeswoman said the malware was instead likely added by a "distributor, transporter, or reseller".

Details of the infections were made public last week.

Microsoft said the criminals behind the malicious program had exploited insecure links in supply chains.

The viruses were discovered when Microsoft digital crime investigators bought 20 PCs, 10 desktops and 10 laptops from different cities in China.

Among the threats discovered was Nitol, a program that tried to steal data from infected machines as soon as they were first turned on.

The company's investigations led them to a domain name - 3322.org - which was believed to have been running 500 separate strains of malware to fool victims or steal data.

A US court has given Microsoft permission to seize control of the domain.