What is the difference between a "recommended" update and an "optional" update from Windows? As of this week, "recommended" now means that Windows 10 is going to install itself on the PCs of most of its users.

If you've been holding off on updating your Windows 7 or Windows 8.1 PC, Microsoft could be about to take that decision out of your hands. In its defence, the company gave users plenty of warning.

Back in October Terry Myerson, Executive Vice President, Windows and Devices Group issued the warning in a blog post: "Early next year, we expect to be re-categorizing Windows 10 as a "Recommended Update." Depending upon your Windows Update settings, this may cause the upgrade process to automatically initiate on your device."

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As it turns out "early next year" began on Feb. 1. Microsoft has released the following statement announcing the change: "We are committed to making it easy for our Windows 7 and Windows 8.1 customers to upgrade to Windows 10. We have updated the upgrade experience today for some of our customers, who had previously reserved their upgrade, to schedule a time for their upgrade to take place. Users are fully in control of their devices."

The automatic download will affect anyone who's Windows PC has have enabled the "Give me recommended updates the same way I receive important updates," which Microsoft urges users to keep for security update reasons.

Not everyone is pleased with Microsoft's "recommended" approach, which some find intrusive or plain confusing.

"I had every intention of upgrading to Windows 10, but at this point I think I'm dragging my feet out of spite because of the way that Microsoft is conducting themselves," wrote one user on Reddit's Windows 10 subreddit.

There are many reasons why Microsoft is keen to get users onto Windows 10, not least because its Bing search engine is deeply embedded in the new operating system and its most recent earnings report showed Microsoft generates about 30 per cent of search revenue from Windows 10 devices.

Microsoft doesn't comment on its Windows install base, but industry estimates place it at more than a 1 billion PCs, and calculations from web analytics firm Net Applications suggest roughly 50 per cent of those run Windows 7, with maybe 13 per cent for Windows 8 and 8.1 and another 11 per cent still running XP. Windows 10 is on about 11 per cent of machines, which according to Microsoft represents about 200 million machines.

The reviews for Windows 10 aren't nearly as negative as 2012's Windows 8, and voluntary adoption has been much more rapid. Importantly, 10 is a free update in its first year, unlike previous Microsoft operating system changes.

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Many savvy Microsoft customers have already disabled recommended updates to avoid Windows 10. If you're a Windows 7 or Windows 8 user, odds are your machine has already added some updates necessary to install Windows 10. If you want to uninstall all of those unnecessary files the folks at Ghacks have compiled a handy list of where to go and what to delete.

But, as Mr. Myerson also wrote back in October, users won't be stuck with an automatically installed Windows 10 update:

"After any upgrade, you can easily go back to your prior version of Windows within 31 days if you choose. Go to Settings->Update and Security->Recovery and Uninstall Windows 10" to return to your prior version of Windows."