Windows 10 - the latest edition of Microsoft's operating system complete with a brand new web browser - launches overnight.
The software will work across all Windows-powered devices for the first time - from smartphones and tablets to laptops and the Xbox One console.
Edge replaces Internet Explorer as the built-in web browser, while the smartphone voice assistant known as Cortana will appear on desktop computers for the first time.
Users were angry when the Start menu was removed from Windows 8, and now it will return in the new software.
Millions of PC users who already use Windows 7 or 8 will also be able to upgrade for free. There was no Windows 9.
Chief executive Satya Nadella says he wants Microsoft to move from "people needing Windows, to choosing Windows, to loving Windows".
The roll out will begin in the early hours of Wednesday morning, and continue over the next few days. Users will be notified on the desktop when their download is ready.
Those not eligible for the free update will have to pay £99 for the home version of the software.
Microsoft has hundreds of millions of Windows users, but its market share is being squeezed by Apple's OS X.
