It is the engine behind the largest personal fortune ever amassed. All-conquering but largely unloved, it has changed the daily lives of hundreds of millions — but Microsoft Windows, the Genghis Khan of computer operating systems, faces an uncertain future.

Vista, the latest version of Windows, was released for businesses last month. The consumer version comes out next month. The revamp of its biggest cash cow is being billed as the most important event for Microsoft in a decade. Windows accounted for $10 billion (£5.1 billion) of the group’s $16.5 billion operating profit last year. Five years in the making, Vista’s 50 million lines of code have cost an estimated $7.5 billion to assemble.

Yet already the knives are out for Vista, a system that Microsoft executives admit will be the last of its kind, as their company finally gets to grips with the internet age. Vista is meant to be slicker and safer than its predecessors, but even after a two-year delay it is “not really ready”, Michael Silver, an analyst at Gartner, said. Companies hoping to switch in the past couple of weeks could not get hold of the additional software needed for it to work with their printers.

Security experts acknowledge that Vista is the most secure operating system that Microsoft has made, comparable to Apple’s latest version of its rival OSX system, but they also note that several flaws have already been uncovered and predict that Vista’s reach — 200 million people are expected to use it in a year’s time — will work to its disadvantage.

“The bad guys will always target the most popular systems,” Mikko Hypponen, of F-Secure, the security group, said. “Vista’s vulnerability to phishing attacks, hackers, viruses and other malicious software will increase quickly.”

The Vista security headache is not the only chink in Microsoft’s armour. Like all the Windows family, Vista traces its heritage back to the MS DOS system that Microsoft developed in 1981, when computers were standalone boxes. Vista is the most web-friendly Windows yet and Microsoft is placing a huge onus on its improved internet search engine, but industry observers suggest that Windows never really forgot its roots and has floundered since PCs became joined-up through the internet.

The fear is that rivals will use the web to kill Windows. Google, a child of the online era, is the No 1 threat.

“Microsoft is way behind Google when it comes to the internet,” Rupert Godwins, the technology editor at ZDNet, the industry website, said. “Building Vista, Microsoft is still doing things the old way at the same time as it undergoes a big shift to catch up.”

Rumours of an impending Google operating system, “GoogleOS”, have proven wide of the mark so far, but the Google Docs & Spreadsheets product does much of what Microsoft’s second-most important product, the Office suite of software, does. Google’s offering is a free, web-based service at docs.google.com. Microsoft’s Office Professional 2003 is much more sophisticated, but it costs as much as £460 and has to be installed on to the hard drive of a PC.

Crucially, the Google word processor and spreadsheet package does not need Vista.

Microsoft is retaliating with its own web-based services, dubbed Windows Live, but the strain of transition has shown inside the world’s largest software group. Jim Allchin, the 16-year Microsoft veteran who ran much of the Windows division and was dubbed the “Vista Godfather”, will leave the company in weeks. In the next 18 months Bill Gates, who became the world’s richest man, worth an estimated $53 billion, on the back of Windows licences, will drop out of the day-to-day running of the company that he co-founded 30 years ago. The changes have put an outsider, Ray Ozzie, the designer behind Lotus Notes and an internet expert, into the key role of chief software architect, Mr Gates’s former title.

Once installed in the post, Mr Ozzie wrote an internal company memo that mapped out the challenges that face Microsoft. The message was clear: get Google, get with the internet and wean Microsoft off Windows as we know it.

“Through Google’s focus they’ve gained a tremendously strong position,” he said. “[Microsoft] must respond quickly and decisively . . . It’s clear that if we fail to do so, our business as we know it is at risk.”

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