Microsoft has launched its new Windows 8 operating system, which the company says is the biggest update to the platform in 17 years.

The new software dispenses with the familiar Start button and features square tiles for apps.

It is designed to work on both PCs and tablet devices.

Head of Microsoft's Windows unit, Steven Sinofsky, said: "This is not just the evolution of computing, it is the revolution of computing."

Microsoft has also launced a new tablet in an attempt to compete with its biggest rival Apple.

The Surface tablet will run Windows 8 and comes with a detachable keyboard.