Editor's note: This article was originally published in April. It has been updated to reflect the official rollout of the Get Windows 10 campaign on June 1.

What was already an open secret has now been confirmed. Microsoft will launch Windows 10 on July 29. (Previously, Microsoft had only committed to "this summer" as a launch date.) That date begins a one-year window during which free upgrades will be available to anyone running Windows 7 or Windows 8.1.

The first phase of the long, slow rollout begins today, with the appearance of a small icon in the taskbar on eligible systems. Here's what it looks like on a PC running Windows 8.1:

Clicking that link opens a six-screen advertisement, with a prominent "Reserve your free upgrade now" button. Clicking that button configures your PC to download and install the upgrade bits and adds the current device to the queue for Windows 10.

But what will actually happen when July 29 rolls around? That poses some interesting logistical questions for Microsoft.

The Get Windows 10 program is going to be one of the largest software delivery projects in history. Microsoft is offering full, free upgrades for every PC currently running Windows 7 Service Pack 1 or Windows 8.1 (excluding those running Enterprise editions, which don't qualify for the free upgrade).

That means a massive bundle of software downloaded to each PC. The official upgrade offer warns that the approximate file size is 3 GB, although for most upgraders the actual package will be slightly smaller.

So just how big is the eligible Windows 10 upgrade base? It is certainly measured in the hundreds of millions, representing PCs running Windows in 111 languages and 190 countries worldwide.

Apple's been doing this for a few years with OS X, but on a much smaller scale, measured in the low tens of millions for each new release. Microsoft's upgrade program for Windows 8.1 was probably larger than that but still only a small fraction of the worldwide PC installed base.

There's actually a road map hidden in plain sight, included with the update for Windows 7 and Windows 8.1 that enables the Get Windows 10 icon. An XML file installed with that update contains important clues about the program.

KB3035583 describes itself innocuously enough: "This update enables additional capabilities for Windows Update notifications when new updates are available to the user. It applies to a computer that is running Windows 8.1 or Windows 7 Service Pack 1 (SP1)."

That's it. No, seriously, that's the entire description. But the KB article does go on to describe the files included with this update, most of them containing the acronym GWX.

And as the file details make clear, GWX is short for Get Windows 10.

The update also sets up four scheduled tasks, one of which runs an "appraiser" app that checks prerequisites for the download and subsequent upgrade. And in the GWX folder is an XML file that contains a roadmap of the different phases in store after that update is installed.

The first salvo is the "Anticipation UX." That's the phase that began today with the appearance of the Get Windows 10 taskbar icon.

The next phase is labeled the Reservation Page. That's the opt-in page, where someone seeing this sequence of ads can read about the process and then say yes, they're ready to upgrade.

The phases after that are mechanical: Upgrading, Download In Progress, Download Complete, Ready for Setup, Setup in Progress, Setup Complete.

The whole process is similar to the flow of events that members of the Windows Insider program are already familiar with: opting into an update that enables delivery of the upgrade files, which are downloaded in the background and then installed with minimal user intervention.

I've seen a handful of people trying to spin this update process as a sneaky move on Microsoft's part, but I have a hard time seeing this as adware. It is, instead, perfectly targeted advertising, offering a free upgrade to a product currently running on the system where the ad is being displayed.

There are no hidden costs (aside from those that might be incurred by the large download for anyone who has a pay-by-the-megabyte data plan) and the upgrade isn't going to be installed without your explicit consent. It can't, because there's at least one license agreement (and several additional confirmation screens) that you're going to have to click through.

This update will be installed automatically on Windows 8.1 machines that are configured to accept Recommended updates. It is an Optional update on Windows 7, so Windows 7 users will only see this offer if they go to Windows Update and manually install this update. The upgrade offer is not being displayed on Enterprise editions or on domain-joined machines, where network administrators typically manage operating system upgrades.

So what happens on July 29?

On or before that date, Windows Insiders will get a Preview build that will become the Current Branch. Think of this as the replacement for what would previously have been a Release Candidate. They will automatically be upgraded the the release version when it is available.

Anyone running Windows 7 or Windows 8.1 has the option to reserve their place in the Windows 10 upgrade queue, starting today. Those upgrades will begin rolling out in late July and early August, staggered to reduce the load.

And as the population of upgraded PCs grows larger, a new feature in Windows 10 will kick in, reducing the load on Microsoft's servers. Options in Windows 10 will allow peer-to-peer transfers of apps and OS updates, on the same network and potentially over the Internet. This feature won't affect the initial Windows 10 upgrade, but it should help with the large OS updates due to arrive on Windows 10 PCs at regular intervals after the initial upgrade is complete.

Of course, while this public release is rolling out slowly, over a period of several months, Microsoft will continue to deliver new feature updates. We already know that the new unified OneDrive sync client is likely to appear after the official release of Windows 10. There will probably be several of these minor feature updates between this summer's Windows 10 launch and the rollout of the first big feature update a year later, code-named Redstone.

The Microsoft Edge browser (still branded with its Project Spartan codename in current builds), the new Mail and Calendar apps, and anything else delivered through the Windows Store can also be updated on their own schedule, independent of the rest of Windows.

When all is said and done, Microsoft will be supporting Windows 10 users in four separate rings: Insider Fast and Insider Slow will continue to get early access to updates; the Current Branch will be the public release channel; and a new Current Branch for Business will allow businesses to delay feature updates by several months, so that they can minimize the risk that a flawed update will have an impact on business processes.

The big question is how many consumers and small businesses will say yes to the free Windows 10 upgrade, and how quickly. Several recent data points suggest that close to nine in 10 PCs running Windows 8 have updated to Windows 8.1. Getting Windows 7 holdouts to upgrade might not be as easy a sell.