Microsoft has launched Windows 8, the centrepiece of one of the computer giant's most ambitious endeavours in its 37-year history.
Addressing those gathered at the Pier 57 building on the banks of the Hudson River in New York City, Microsoft head of Windows, Steven Sinofsky, said: "Windows 8 is simply the best release of Windows ever."
He hailed it as a major milestone in the evolution of computing.
Designed to run on PCs, tablets and a yet-to-be released line of smartphones, Windows 8 heralds the biggest change to the industry's dominant operating system since the emergence of Windows 95.
Mr Sinofsky said Windows 8 had an improved battery life, quicker boot time and had been subject to 1.2bn hours of testing.
Windows 8 attempts to bridge the gap between personal computers and fast-growing tablets with its touch-enabled interface.
Microsoft's launch comes amid a slew of other tablet offerings, with Apple unveiling its iPad Mini with a 7.9in (20cm) screen on Tuesday .
Amazon and Barnes & Noble, makers of the 7in (17.7cm) Kindle Fire and Nook Tablet, are coming out with larger versions next month.
At the same time as launching the new Windows operating system, Microsoft is also introducing its Surface tablet - its first venture into making computer devices.
That goes on sale on Friday, along with computers and other tablets running Windows 8.
"I think Windows 8 is a great operating system for the tablet," hedge fund analyst Steve Mackay told Sky News at the Microsoft launch.
"I think it's going to be better than Android. I think Apple is still going to sell the lion’s share, but I think (Microsoft) has a real chance of being a legitimate player in the tablet market."
One factor that might dampen enthusiasm for the Surface is its price - $599 (£372) with its touch keyboard cover - and its availability for purchase only from a limited number of Microsoft stores and online, said analyst Kirk Adams.
He said consumers may be reluctant to buy the device until they can try it in person.
Meanwhile, most analysts believe companies and governments will hold off on upgrading to Windows 8 for at least another year.
About half of business users still have not upgraded to Windows 7 from Windows XP, which came out in 2001.
XP also marked the last time Microsoft had a huge launch campaign for Windows.
Releases of Windows Vista and 7 since then have been more subdued.