Business Insider
Terry Myerson, Microsoft's EVP of operating systems.
That's big news. Microsoft typically charges hundreds for new versions of Windows.
It should also help quickly drive adoption for the new operating system. Windows 7 is the most popular version of Windows, even though it's several years old. Users have been hesistant to upgrade to Windows 8 since it has a radically different interface.
Windows Phone 8 users will also be able to upgrade to Windows 10 for free. Windows 10 is designed to work on all Microsoft gadgets: phones, PCs, tablets, and even the Xbox.
Windows 10 takes a step back from Windows 8 and includes an interface that's easier to use on traditional desktops and laptops.
The free offer upgrade is only good for the first year after Windows 10's debut. Microsoft hasn't announced a launch date yet.
Follow the rest of our live coverage of the Windows 10 announcement here.