(Telecompaper) Microsoft has unveiled the latest version of its Windows operating system, designed to work across PCs, notebooks, tablets and smartphones. 'Universal' apps will work across any Windows device, so users can pick up where they left off when changing from computer to phone. Microsoft will offer customers already using Windows 7, Windows 8.1 and Window Phone 8.1 a free upgrade to Windows 10 the first year after the OS launches. Windows 10 also represents the start of 'Windows as a service'', with Microsoft promising to keep the OS up to date for the life of a customer's device, at no extra charge. Other new features include the integration into Windows of Cortana, the personal assistant software already available for Windows phones, and the launch of a new web browser, with the code name 'Project Spartan'. Microsoft has been testing the new Windows since last September, through its Windows Insiders programme. In the next week, the company will release to Insiders the latest build for the Windows 10 Technical Preview on PCs. In February, the first technical preview for phones will be available. The launch of the final version is expected later in 2015.