Microsoft's Windows 8 is the most radical reinvention ever of the PC operating system used by two billion people worldwide.
The new version is built both for 'traditional' PCs and touchscreens in a relaunch that has reportedly cost $1 billion.
The 'new look' is a total 'reboot' for Windows - adding a new, simpler look, and getting rid of the famous 'Start' button.
Owners of Windows 7 machines can upgrade to the new operating system from October 26 for £24.99.
Microsoft is now selling its own £499 'Surface' touchscreen - a direct rival to iPad, but with a clip-on magnetic kebyoard on sale from October 26.
Other manufacturers such as Samsung, Toshiba and Sony are selling machines built for the new system - ranging from iPad-style tablets to 'normal' PC laptops and desktops.
The stars of Windows 8 could be hybrid machines, such as Dell XPS Duo 12, which convert from laptop to tablet and back again, like iPads for grown-ups.
Unlike the operating system of iPads and Android devices, the 'slates' are much more than icon buttons - they deliver information such as weather forecasts, emails and pictures direct to the desktop.
So far, tech reviewers are not 100% sold. Jason Jenkins of CNET says, 'Windows 8 is a huge departure , aimed at heading off the competition from tablets, especially Apple's iPad.
"The result is something that is part modern-day tablet, part old-fashioned corporate Windows, and I'm not sure it truly succeeds at either. From Microsoft's point of view, this has to work. If it fails, its dominance of the computer industry is over."
Can I use my old PC?
Many Windows 7 machines can upgrade - Microsoft lets you check yours at a special page. You'll have to pay to download the new OS.
How much will it cost?
Owners of Windows 7 machines can upgrade for £24.99 for a downloadable copy. If you bought a Windows 7 machine after June 2, it's £14.99. The first Windows 8 machines are quite expensive - Microsoft's Surface is £499, more than iPad.
What's the big change?
The new look: all your apps show up as 'tiles' on screen, which works better with touchscreens. Many Windows 8 devices have both keyboards and touch panels.
What happened to the Start button?
It's gone, but its functions are still there: right-click in the bottom corner, and a simplified menu pops up.
Can I use it with a keyboard and mouse?
Yes. Microsoft's new OS is a 'hybrid', and has two 'looks' one more similar to the traditional Windows, and built for use with keyboard and mouse.
How do I switch between the two 'looks'?
The Windows button on your keyboard switches between the two. In 'traditional' Windows mode, you have more detailed control over apps, where the tiled view is better for touch control.
What's the point of hybrids?
On hybrid machines, you have a keyboard for 'proper' computing with apps such as office, but can still prod at the screen for touch control - or slim it down for use as a tablet on the go.
What are the benefits?
Windows 8 has been slimmed down, and starts faster. Early reports claim it runs faster and consumes less battery than Windows 7.
What are the drawbacks?
Many people find the 'dual' operating system - the 'slate' look, and the desktop look exist side by side - confusing, and switching between the two fiddly. For tablet users, it's much less simple than iPad.
What apps can I use?
Windows 8 will see the launch of Windows Store, which will offer apps for the new system. Like Apple's - but unlike Google's - it will be policed, making it easier to find safe, good apps.