Windows 8 marks the biggest single strategy change in Microsoft's 37-year history, and the most ambitious change in the software that has run on 1.5 billion machines worldwide since Windows 95.
At a launch event yesterday in New York, Microsoft head of Windows Steven Sinofsky said that Windows 8 "is simply the best release of Windows ever".
He said that the operating system has improved battery life, has sped up boot time and was subject to 1.2bn hours of testing.
Retailers around the world are now selling desktop PCs, laptops, tablets and hybrid devices with the touch-enabled Windows 8 installed.
Windows 8 is available to buy as a standalone download for £24.99 in the UK. The software can also be purchased pretty much anywhere that computer equipment is sold, including Currys currently offering the boxed DVD for £49.99 online.
Microsoft's Windows Upgrade Offer means that anyone who purchased a Windows 7 PC between June 2, 2012 and January 31, 2013 can download the Windows 8 upgrade for just £14.99 in the UK.
Microsoft hopes to fend off renewed competition from Apple's Mac by offering a touch-based, dynamic operating system that can run on processors designed for high-spec machines and chips that usually run on mobile devices.
The new operating system borrows heavily from Microsoft Windows Phone software, including the use of 'live tiles' - squares laid out in different sizes on a loose grid, denoting each different app or programme.
These 'tiles' launch the applications, but also show updates in real-time, such as the 'Skype for Windows 8' app alerting the user when they have a missed call. Users can also switch to the traditional Windows desktop view if they so wish.
Windows 8 is the official product name for the software on PCs running x86 processors - 32-bit and 64-bit. There are two versions available: Windows 8 and Windows 8 Pro, rather than the previous system of Home Basic, Premium, Pro and Ultimate versions for Windows 7.
For ARM-based tablets and PCs, which generally pack more battery life but lower processing power and are often referred to by Microsoft as 'Windows on ARM' (WOA), there is a different version of the operating system, known as Windows RT.
Windows RT runs on the initial range of Microsoft Surface tablets that are now available to buy, but Microsoft intends launch slates running Windows 8 Pro next year.
From today, some existing PC and laptop owners can download a Microsoft programme that installs Windows 8 over their existing copy of Windows 7, Windows Vista or Windows XP.
Windows 7 users can transfer all their settings, files and programmes over to Windows 8, while Windows Vista (Service Pack 1) users can only move settings and files, and those on Windows XP (Service Pack 3) can just transfer files.
Anyone with Windows 7 Professional or Ultimate on their machine right now is advised to go for Windows 8 Pro, but everyone else will find the Windows 8 version meets their needs.
Microsoft is giving away the Windows Media Centre pack for free to Windows 8 Pro customers until January 2013.
As a minimum system requirement to run Windows 8, your machine must have a 1GHz or faster processor; 1GB RAM for 32-bit, 2GB RAM for 64-bit; and 16GB of available disk space for 32-bit, or 20GB for 64-bit. You also need a 1366 x 768 screen resolution and DirectX 9 compatible graphics.
Watch a video introducing the Surface tablet below: