microsoft Windows 10 with holographic headset :The future is here .

Microsoft has revealed that Windows 10 will bring its voice-controlled assistant Cortana to PCs.

It also unveiled a headset that it said would one day project the operating system over views of the real world.

In addition, the firm announced that the OS upgrade would be offered free of charge for devices running Windows 8, Windows 7 and Windows Phone.

The offer, which is limited to the Windows 10's first year of release, may aid its adoption.

It marks a change in strategy from Microsoft's previous policy of charging for major updates, and could help avoid a repeat of the relatively slow uptake of Windows 8.