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In a few weeks, Microsoft will deliver the Anniversary Update to Windows 10, marking the third major release of its flagship OS in one year.

By traditional Windows standards, that's practically unheard of. But, then again, much of what's happened in Redmond during the last year would have been unthinkable just a few years ago.

For this post, I looked back at some of the major milestones along the way, with a few lessons that Microsoft and its customers have learned.

The Windows Insider Program changes everything

Windows boss Terry Myerson gets noticeably animated when he talks about the Windows Insider Program. "The Insider program has changed how we do everything," he told me in a recent interview. "It's changed how we build Windows and how we interact with customers and fans."

He's right. Previous releases of Windows usually spent years in private testing before finally rolling out to the public in limited preview releases. Windows 10 radically changed that model, allowing anyone to sign up for access to frequent (sometimes several times in one week) preview builds. It also, crucially, provided a public feedback mechanism, with opportunities for other Insiders to add a +1 to feature requests and bug reports.

In fact, the Insider program is just one facet of an even larger shift in Windows, with different populations able to choose the bleeding edge (Insider Fast), or a conservative, enterprise-focused release schedule; the Current Branch for Business, available for Windows 10 Pro and Enterprise editions delays feature upgrades several months compared to the Current Branch that the general public sees.

Windows 10 is certainly a response to the Windows 8 debacle, with a user experience that is less bold (in a good way) and a feedback loop that's considerably less arrogant.

Microsoft stumbles addressing privacy fears

One of the biggest changes in the "Windows as a service" model is the reliance on telemetry, which involves collecting anonymized information about how people use Windows and, critically, when and where the operating system fails.

The goal is certainly laudable. Microsoft has been criticized in the past for too many releases of defective patches and updates. With accurate real-time data about crashes and hangs, it should be possible to find and fix problems far more quickly than in the past.

A default installation of Windows 10 sets telemetry collection to its highest level, which provided ammunition to the small but vocal legion of Microsoft haters (and some clueless analysts), who insisted that Microsoft had built a keylogger into its new OS and was reading all your files.

None of that was true, but the company didn't help its cause by dragging its feet for months on formal documentation for the feature and by delivering public statements that were too cautious and legalistic.

In an interview this week, Windows VP Yusuf Mehdi acknowledged that "Listening to customers and how we communicate is probably the highest-order bit -- the biggest example of how we learned. As we started to do all of these rollouts, people were saying, 'Tell me about the privacy policy, tell me about telemetry. What are your principles?'"

Today, he says, "We feel great about the [privacy] policy," but admits, "I don't expect there will ever be a day when all people say it's perfectly fine. There will always be feedback, and we'll listen to it."

Version 1511 gets down to business

In one sense, the first feature update for Windows 10 was a proof of concept. Version 1511, delivered in November 2015, was the first-ever full-version upgrade delivered via Windows Update. Arriving only about four months after the initial release of Windows 10, it was filled with bug fixes but also included major new features.

Most of those features were business-focused, but none was more important than Windows Update for Business, which gave enterprise administrators the option to delay updates by up to four weeks and defer upgrades by up to eight months.

"Enterprise adoption picked up after [version] 1511," Myerson told me, adding that the US Department of Defense's decision to adopt Windows 10 on four million desktops was "a real milestone for us".

According to Microsoft's stats, 96 percent of enterprise customers have Windows 10 pilot programs going on. Actual adoption of Windows 10 in the enterprise, however, is still a tiny number. That reflects the traditionally conservative outlook of those customers and represents both an opportunity and a threat to Microsoft, which doesn't want to see Windows 7 turn into another XP, hanging on well past its sell-by date.

Surface bugs highlight the dilemma of the hardware ecosystem

Last October, a few months after the launch of Windows 10, Microsoft unveiled two members of its flagship Surface line. Within days, owners of the Surface Book were reporting serious bugs, and it was several more months before a series of driver and firmware updates fixed most of those issues.

Those issues also affected the Surface Pro 4, which shipped at the same time.

Both devices were designed to be a showcase for Windows 10. As I wrote after six months of using a Surface Pro 4, though, "I can report that it does indeed show off Windows 10, including its occasional frustrations and aggravations."

Most of the problems that plagued those early adopters have now been fixed, and based on my experience with devices from other manufacturers, Intel's Skylake processors can shoulder much of the blame.

Since last fall, Myerson notes, PC OEMs have released more than 1,500 Windows 10 devices. That diversity is both a strength and a weakness for the Windows platform, which has to support millions of hardware configurations. I'm hearing far fewer hardware-related complaints these days, and the latest Anniversary Update preview release is running perfectly on Surface devices here. (Knock on wood.)

Free upgrades mean impressive momentum

Over the past year, Microsoft has not been shy about announcing milestones in Windows 10 adoption. After one month, it had 75 million active users.

In January, Windows 10 crossed the 200-million threshold. By May, it was 300 million, and the most recent figure, released more than a month before the end of the free upgrade offer, was 350 million.

By the numbers, at least, that makes Windows 10 a success. Most of those devices are being used by consumers, of course, not businesses, but Myerson says customer satisfaction is "at an all-time high".

Momentum on the Windows 10 app front hasn't been as impressive, at least by the numbers. Microsoft points to high-profile app releases like the new Netflix client, but the reality is that the Universal Windows Platform is still caught between the mobile iOS/Android juggernaut and legacy desktop apps.

So, what happens when the free upgrade offer expires? Watch those momentum-based press releases for your answer.

The market for Windows phones collapses

Windows 10 Mobile is a full-fledged member of the Windows family, built on the same core code and capable of running many of the same universal apps that run on desktop version. The Anniversary Update will be available for Windows 10 phones soon after it ships for PCs.

The trouble is, there aren't very many of those phones in the wild, and the supply is shrinking. Although the user base for the platform is enthusiastic, Microsoft management telegraphed its plans weeks before the official launch of Windows 10, with a $7.6bn write-down and massive layoffs in the phone division.

The other shoe dropped in May 2016, with CEO Satya Nadella announcing "We are focusing our phone efforts where we have differentiation -- with enterprises that value security, manageability and our Continuum capability, and consumers who value the same."

Those devices haven't yet appeared, and the long-rumored Surface Phone shows no signs of emerging from the mists anytime soon.

Meanwhile, the company's efforts on other platforms have increased substantially, with dozens of apps for iOS and Android appearing over the year, most of them based on cloud services like OneDrive and Office 365.

It's not just about PCs

The phone business might be moribund, but there's still optimism around other hardware coming from Redmond.

Windows 10 powers the Xbox One, which will get its own Anniversary Update this summer, along with a new, smaller console. Xbox sales have been steady, if not spectacular.

The company's acquisition of Minecraft might turn out to be a hit as well, with new versions for Windows 10 on PCs and for Xbox.

But the really big bet is HoloLens and the "mixed reality" holographic platform, both powered by Windows 10. I had a chance to experience the HoloLens experience under controlled circumstances at this year's Build conference. It's genuinely exciting stuff.

Some big questions face this new, untested, rapidly evolving category, though: Can it become a sustainable business? And if it can, how long will it be before those big R&D expenditures pay off in revenue?

GWX shows Microsoft's annoying side

I first discovered the Get Windows 10 app in April 2015, long before it made its public debut.

Microsoft designed the program as a way to roll out Windows 10 to the hundreds of millions of Windows machines that qualified for the free upgrade program. For the first few months it was alternately convenient and annoying, depending on whether you were ready for the upgrade.

And then it went too far, with the notifications becoming more persistent and, in some cases, with the GWX program downloading several gigabytes of Windows installation files in the background without the user's knowledge. Some users reported that the upgrade began without their consent. Was that a bug or a feature? Microsoft remained stubbornly quiet in response to most of those complaints.

The GWX program was constantly shifting its behavior throughout the first year and frustratingly never offered an easy option to decline the upgrade offer. It took either registry edits or third-party software to deliver an unconditional "No, thanks".

Thankfully, the GWX program will expire on July 29, 2016, when the free upgrade offer ends. How long will it take for the ill will caused by the GWX program to vanish? Sadly, that one has no expiration date.

Three major releases in one year is a record pace

The Anniversary Update for Windows 10, version 1607, should be signed off soon, perhaps even before this week is out, in time for its public release on August 2.

I'll have a detailed look at what's new as that version gets closer to its official release, but for now, it's probably more important to note that this is a major upgrade, with big changes that improve the Windows 10 user experience without adding undue confusion. (Apparently the lesson of Windows 8 sunk in.)

That makes three major releases in a one-year period, which is faster than Microsoft has ever moved before.

For businesses, this update will probably be the equivalent of Service Pack 1-- the signal that it's OK to begin deployments rather than remain in pilot mode.

The road to a billion hits a few speed bumps

In April 2015, Terry Myerson drew a line in the sand, predicting that "Windows 10 will be installed on 1 billion devices within two to three years".

I did the math on that claim a few weeks later and said it was realistic. But my numbers relied on Windows Phone continuing to sell at least 50 million handsets per year, in addition to upgrades, for a total of at least 200 million Windows 10 Mobile devices in the market by 2018. (Follow that link to see my original analysis.)

That's not going to happen. And, meanwhile, the traditional PC market continues to shrink, slowly.

Add those two factors together and you get a longer ramp-up, which Microsoft officially confirmed to me this week, with a statement from Yusuf Mehdi:

Windows 10 is off to the hottest start in history with over 350 million monthly active devices, with record customer satisfaction and engagement. We're pleased with our progress to date, but due to the focusing of our phone hardware business, it will take longer than FY18 for us to reach our goal of 1 billion monthly active devices. In the year ahead, we are excited about usage growth coming from commercial deployments and new devices -- and increasing customer delight with Windows.

Although Myerson and his team are still confident about their ability to hit the 1-billion milestone, it's unlikely to happen by 2018 as originally projected.

Note: This post was updated about 10 minutes after its initial publication with a statement from Microsoft about its "1 billion devices" goal.