Users on Windows 7 or Windows 8.1 will be able to upgrade for free within a year of Windows 10's launch on July 29th. That leaves users on Vista or XP needing to pay for the new operating system. However, yesterday Microsoft revealed that if you install Windows 10 preview (build 10130) using a Microsoft Account that's registered for the Windows Insider program, you'll receive the final version of Windows 10 for free.
Windows Insiders running the Windows 10 Insider Preview (Home and Pro editions) with their registered MSA connected to their PC will receive the final release build of Windows 10 starting on July 29th. This will come as just another flight. I’ve gotten a lot of questions from Windows Insiders about how this will work if they clean installed from ISO. As long as you are running an Insider Preview build and connected with the MSA you used to register, you will receive the Windows 10 final release build and remain activated. Once you have successfully installed this build and activated, you will also be able to clean install on that PC from final media if you want to start over fresh.
More details can be found in the Microsoft blog post linked below...
Read More [via Verge]