Microsoft's Windows 10 officially launched overnight, although many users will not get it straight away.
The new operating system is being gradually rolled out and is available in 190 countries.
It replaces the last version which was criticised for the way the desktop looked.
Windows 10 will be free of charge for existing users of Windows 7 and 8 devices - and sees the return of the 'Start' button.
However, you must upgrade to Windows 10 within one year of its availability.
Microsoft says it has "more than 2,000 devices or configurations already in testing."
"A new era of Windows starts today. From the beginning, Windows 10 has been unique - built with feedback from over five million fans, delivered as a service and offered as a free upgrade," said Terry Myerson, executive vice president of Windows and Devices Group at Microsoft.
"Windows 10 delivers on our more personal computing vision, with a natural, mobile and trusted experience. Along with our partners, we're excited to deliver the best Windows ever, which will empower people and organisations around the world to do great things."
Joe Osborne from Tech Radar in the US says Microsoft is not charging most users, as its strategy has changed.
