Windows 10 with Cortana
Cortana sits on the desktop of Windows 10 and can be spoken to to write emails, search the web, launch apps and moreScreenshot

Windows 10 brings Cortana, Microsoft's voice-controlled person assistant, to desktop computers and laptops for the first time.

Demonstrated at a media event this week, Cortana can look up information online, check your calendar, send messages, track your upcoming flights, compose emails and more with spoken instructions.

An obvious rival to Google Now and Apple's Siri assistant, Cortana launched on Windows Phone 8.1 devices last year, but promises to do more and feel more natural on Windows 10. The assistant starts listening as soon as you say 'hey Cortana' and then she can give weather forecasts, set reminders, and even predict sports scores, like next week's Super Bowl.

Speech recognition is smart enough to know when you mention contact's name, and can even understand hash tags, adding # to the start of a word of phrase, rather than typing out the two words.

Cortana's knowledge comes from Microsoft's Bing operating system, but can also learn your behaviour from recent searches and the new 'Notebook', which catalogues everything Cortana knows about you.

But her knowledge of you can run much deeper. Cortana watches you as you browse the web, bringing up information based on what you're looking at. Click on an ingredient of a meal on a restaurant menu and Cortata (which also runs in Microsoft's new Spartan web browser) appears on the right side of the screen to explain what it is, plus dietary information.

As well as searching the web, Cortana can search for files stored on your computer, and for self-concious users who don't want to look like they're having a conversation with their computer, Cortana responds equally well to written instructions and search requests via a box next to the Start button.

Cortana is where you will find your notifications, such as unread emails, reminders and upcoming calendar events, and voice commands can also be used to control music playback and launch apps.

Finally, for reasons best known to Microsoft itself, Cortana can speak in Yoda's voice. We're not really sure why, either.

Appearing across phones, tablets and PCs running Windows 10, Cortana can also remind you where you parked your car with the help of Windows 10's new mapping app.

Microsoft's Joe Belfiore said Cortana will roll out gradually over the coming months, and it'll be introduced across several countries during that time.