The new 'Windows 8' start screen (Credit: Microsoft)

Early versions of Microsoft’s Windows Mobile, developed a decade ago, had the feel of the Windows PC desktop. The theory was that people would be more comfortable and familiar with it. The upcoming version of Windows for PCs — known for now by the code name Windows 8 — will feel more like a mobile phone or a tablet. Same theory.

The world has gone mobile, and online. And good old-fashioned Windows, that bastion of traditional desktop computing, will undergo a radical remodel in an attempt to keep up with the likes of Apple and Google … and everybody else.

In the process, Microsoft will be taking an enormous gamble, overhauling the centerpiece of its business — a product responsible for more than $18 billion in annual revenue, and familiar to hundreds of millions of people. If the strategy works, it could keep Windows relevant for years to come. If it doesn’t, it could accelerate the company’s decline.

It’s the biggest change since Windows 95, said Microsoft executive Julie Larson-Green yesterday at the D9 conference in California, where she and Windows chief Steven Sinofsky debuted the new approach.

“An average person walking into Best Buy and going to look at a Windows laptop is going to be shocked when they look at that,” said Walt Mossberg, the conference co-host and influential Wall Street Journal columnist. He called it “jolting.”

“It’s definitely different,” acknowledged Larson-Green. “It really takes into account all the changes that have happened in the industry, and all the technologies. While we just showed the user interface here, every subsystem of Windows has been reimagined to be modern.”

And that was the big shocker from the Windows 8 unveiling. Microsoft wasn’t just showing a new Windows for tablets, as many had expected. Microsoft was showing a new Windows … period.

“It’s going to run on laptops, it’s going to run on desktops, it’s going to run on PCs with mouse and keyboard,” says Microsoft’s Jensen Harris after demonstrating the Windows 8 interface in the company video below. “It’s going to run on everything.”

Also significant is Microsoft’s focus on web-oriented HTML5 and JavaScript applications in the new Windows development platform. We’ll hear much more about this at Microsoft’s “Build” developer conference in September.

Users will still be able to run traditional Windows software, and opt for the traditional Windows desktop, but the standard experience will be the new interface shown by the company this week.

The scale of the change underscores just how much Windows needs to adapt to the new world. Particularly interesting during the D9 demo was the brief glimpse of the “share” icon built into a default Windows 8 command menu. This is Windows for the web, more than any previous version of the operating system.

At the outset of the D9 session yesterday, conference co-host and Wall Street Journal columnist Walt Mossberg asked Windows chief Sinofsky how he felt about Microsoft being left out of Google chairman Eric Schmidt’s “Gang of Four” list of top consumer platforms: Amazon, Apple, Google and Facebook.

“The thing that struck me the most was, it’s an interesting group of companies, lots of creative work,” said Sinofsky. “But the one thing thing that gets left out of that discussion is that the way 90 or 95 percent of the world gets on the Internet in the first place is through Windows.”

Microsoft isn’t saying when the new Windows will come out, but it’s widely expected in 2012.

Related: Platform or not, Microsoft’s Xbox business is booming

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