If you are one of those folks who are rocking on Windows Xp or Vista on your machine , Microsoft will allow users of Windows Xp and Windows Vista to get updated to Windows 10. Previously, it was believed that only Windows 7 and Windows 8 users would be eligible for the free upgrade.
On Windows Blog, Microsoft effectively writes that users running Windows preview builds as part of the Windows Insider Program will be able to upgrade to Windows 10 for free.
Microsoft has claimed in the past that users upgrading from Windows 7 and Windows 8.1 will receive Windows 10 for free, however that was never the case for Windows Insiders who installed clean. Microsoft consistently said that you would need an existing Windows 7 or Windows 8.1 license to get activated on Windows 10, but now that seems to have changed. – WinBeta
Windows Insider users must be running the most up-to-date preview release of Windows 10 (build 10130). Once the final version of Windows 10 goes live, these users will be able to download and install it for free.