There has been a lot of confusion over how to get Windows 10 for free when it launches. For now, anyone can get what is technically the beta version gratis. You just need to enroll in the Windows Insider Programme (you will need a MSA or Microsoft account, Skype or Outlook or Hotmail) and download the Windows 10 preview.
However, not only will this version, the insider preview, going to expire on 1 October 2015 but it will also be different from the final version, not least because Microsoft will get rid of all the telemetry tools, thereby improving performance and battery life.
These anonymously monitor usage and send back reports about any issues encountered, ultimately helping Microsoft build a better product using data crowdsourced from more than four million Windows 10 users.
So if you want to keep Windows 10 come October, then you will HAVE to continue using a new build that will come out by then. This means being tethered to Microsoft's telemetry services forever (well almost) and acknowledging that your system might not work optimally.
Therefore, computers that do not qualify for the free upgrade, either because they don't have an original OS or because they run Microsoft operating systems prior to Windows 10, will become beta testing machines for life.
Microsoft confirmed it in writing "Since we're continuing the Windows Insider Program, you'll be able to continue receiving builds and those builds will continue to be activated under the terms of the Windows Insider Program".
A free lunch?
That said, given that the company had a change of mind just a few days ago, things might change again between now and the launch of Windows 10 on 29 July.
Also, just to be clear, Windows 10 will be a free upgrade only for licenses that are not part of any business/volume licensing agreements like Software Assurance or Enterprise Agreement. If you want to install it on more than one computer, then make sure you have registered more than one MSA.