Windows is the most popular PC operating system on the planet today with more machines running it than any other OS, by a long shot. In March, we learned that Windows 10 had been installed 800 million times making it the fastest growing version of Windows ever. 

An internal document seen by Thurrott claims that Windows 10 is now active

Source: Hot Hardware – Microsoft Internal Docs Windows 10 Activations Continue Rapid Expansion To 825 Million