Alas Windows XP and Windows Vista users, you'll have to buy Windows 10 when it comes out.
Previously a blogpost on the official Microsoft website inferred that anyone who had installed a preview of the Microsoft Windows 10 OS would get a free upgrade once it officially launches July 29.
Microsoft has since amended the post and added this reminder: "It’s important to note that only people running Genuine Windows 7 or Windows 8.1 can upgrade to Windows 10 as part of the free upgrade offer."
Currently Microsoft is rolling out updates to those on Windows 7 and 8, reminding them to reserve a copy of Windows 10 when it finally comes out. We'll see whether this version will finally get the holdouts on XP and Vista to finally upgrade their end-of-life software.