Microsoft has unveiled its Windows 8 operating system, describing it as a "major milestone" in the evolution of computing.
Designed to run on both PCs and tablet computers, Windows 8 has been described as the biggest change to the industry's dominant operating system in at least 17 years.
It attempts to bridge the gap between personal computers and fast-growing tablets with a touch-enabled interface.
Windows 8 features a new Start screen that gives one-click access to the apps and content, a new Internet Explorer 10 designed for touch screens and built-in cloud capabilities.
Steven Sinofsky, president of the Windows division at Microsoft, told the US launch that Windows 8 was "computing for the next billion people" and marked the shift from Windows 95 to the age of touchscreens, smartphones and social networking.
He said Windows 8 has a better battery life, faster boot time and a smaller memory footprint and has undergone 1.2 billion hours of testing. More than 1,000 new PCs have already been certified for Windows 8, with some costing less than 300 US dollars, he added. The company is also launching the Surface tablet, its first venture into making computer devices.
Microsoft UK announced that Windows 8 would go on sale at midnight, while the tablet is available to order online from £399.
Jean-Pierre Van Tiel, UK director of Windows, Microsoft, said: "With the launch of Windows 8, Microsoft is unveiling a reimagined Windows to the world. Whether you want a tablet or a PC, whether you want to consume or create, whether you want to work or play, Windows 8 delivers a personalised experience that fits your unique style and needs."
Consumers currently running PCs with Windows XP, Windows Vista or Windows 7 will be able to download Windows 8 Pro from January for an estimated price of £24.99, Microsoft said. Eligible Windows 7 PCs bought between June 2012 and January 31 next year will be able to upgrade for an estimated £14.99, Microsoft UK said.
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.