Computer security experts were grappling with the threat of a newweakness in Microsoft’s Windows operating system that could put hundreds of millions of PCs at risk of infection by spyware or viruses. The news marks the latest security setback for Microsoft, the world’s biggest software company, whose Windows operating system is a favourite target for hackers. “The potential [security threat] is huge,” said Mikko Hyppönen, chief research officer at F-Secure, an antivirus company. “It’s probably bigger than for any other vulnerability we’ve seen. Any version of Windows is vulnerable right now.” The flaw, which allows hackers to infect computers using...