Computerworld confirmed on several Windows 7 PCs that Microsoft had triggered the notification.
The campaign is contained within an application dubbed "Get Windows 10," which includes several screens that trumpet the free upgrade to consumers and small businesses, and let users "reserve" a copy.
Earlier today, Microsoft announced that the Windows 10 upgrade would be available starting July 29, just eight weeks away.
News of the nag campaign first surfaced after Myce.com spotted, then rooted through a March 27 update that was pushed to Windows 7 and Windows 8.1 devices. The update was marked "Recommended" for some machines, meaning it was automatically downloaded and installed where Windows Update had been left with its default settings intact. On most PCs, however, the update was optional.
The campaign -- identified by Microsoft as the KB3035583 update -- was changed to Recommended for all eligible devices on May 14.
Microsoft flipped the Get Windows 10 switch today. According to the scheduled tasks the app adds to Windows, it will appear after logging in, and refresh itself daily at 8 p.m. local time.
A small Windows logo icon appears in the notification section of the task bar -- at the bottom right of the screen -- and clicking the icon launches the Get Windows 10 app.
Currently the only thing users can do is reserve a copy, a process that requires a single click in the app. Anyone who wants a confirmation must also provide an email address.
Between now and July 29, however, the app will download and install other updates that Microsoft said will be necessary "to make the final installation go more quickly." The upgrade files -- about 3GB worth -- will be silently download to the device in the background, on or after July 29, and the user will be told when it's ready to install.
It was unclear if everyone who reserved a copy through Get Windows 10 will receive the all-clear on July 29. "When your upgrade is ready after July 29, 2015, you get a notification that lets you get started," Microsoft said in a long FAQ published Monday.
On Reddit, where reports of the nag campaign first surfaced, many commenters weren't keen on the concept.
"Can't wait for the million #$%@ing tickets I'm gonna have about this icon," wrote someone identified as tribulator22, presumably an IT or help desk administrator.
Users can kill the task bar icon by selecting "Customize" from the task bar's notification area -- the option is under the small up-arrow at the left of the section -- then choosing "GWX" and setting it to "Hide icon and notifications."
Alternately, users can remove the Get Windows 10 app and its marketing push by uninstalling KB3035583 from the Windows Update pane. (On Computerworld's Windows 7 PCs, Get Windows 10 did not appear in the Control Panel section devoted to uninstalling programs.)
When the free upgrade arrives, users will have one year -- until July 29, 2016 -- to grab it. The version served to customers will depend on what they're running now: Windows 7 Starter, Windows 7 Home Basic, Windows 7 Home Premium and Windows 8.1 will receive Windows 10 Home, the consumer-grade edition.
Those with devices powered by Windows 7 Professional, Windows 7 Ultimate, Windows 8.1 Pro for Students and Windows 8.1 Pro will get Windows 10 Pro.
KB3035583 was not pushed to devices running Windows 7 Enterprise or Windows 8.1 Enterprise, the versions used by large organizations and companies. The nag icon and screens will also not appear on Professional or Pro editions when they have been joined to a corporate network.
Non-genuine Windows -- that's a Microsoft term for pirated or counterfeit copies of the OS -- will not see the notifications or the Get Windows 10 app. Last month Microsoft retreated from earlier statements and said that non-genuine Windows was not eligible for the free upgrade.