JANUARY 2018 UPDATE: 

Until recently, canny Windows 7 and 8.1 users could get a free Windows 10 upgrade thanks to a Microsoft loophole. Unfortunately, this offer has finally been closed by Microsoft (as of December 31st 2017), so it is no longer possible to upgrade to Windows 10 for free.

We flagged this loophole in our February 2018 issue of Which? Computing. Unfortunately, as the article was passed for press before the December 31st change by Microsoft, we weren't able to update our advice in time, and we wish to apologise to any readers who have been left disappointed to miss out on the free upgrade.

What was the free Windows 10 upgrade?

There was, until this deadline finally passed, a widely-known loophole that let Windows 7 and 8.1 users bag a free copy of the Windows 10, even after the initial free upgrade offer that ran from July 2015 to July 2016. 

Microsoft continued to make Windows 10 available for free to anyone who needed 'assistive technologies'. That's to say, software or hardware that's designed to help people with disabilities. This could be something as simple as a screen magnifying program, for instance.

Microsoft didn't actively check whether you use such software or hardware when downloading the Windows 10 upgrade file. So, in practice, this loophole made the free offer available for anyone.

It was widely suspected that the software giant would prefer users to take the Windows 10 upgrade for free rather than stay on older operating systems. In the long run, it will cost Microsoft more money to have to maintain support for Windows 7 or 8.1 users, after all.

On Microsoft's official advice page, however, it now states that the loophole has at long last been closed. We will update our advice if any similar promotions to Windows 10 are made available by Microsoft.