Microsoft's latest and greatest operating system Windows 10 launches in 190 countries on 29 July. Here are the 10 most important things you need to know.

1. Windows 10 is free

Well, it's free for anyone with a valid Windows 7 or Windows 8 licence for the first 12 months after it has been released. After that a new licence will cost £99.

However, if you have a Windows 7/8 licence for one machine it doesn't mean you can transfer that licence to new machine that you want to run Windows 10 on.

2. Not everyone will be able to upgrade to Windows 10 on the day of launch

Microsoft is planning a phased roll-out of Windows 10, with those currently testing early versions of the software the first in line to get the update.

Windows 10 will also be available on launch day to those purchasing new computers from the likes of Dell and Lenovo.

Next it will roll out to desktop and laptop users before the mobile version launches in autumn (Q3 2015).

3. Windows 10 will work on all devices

From traditional desktops, laptops, tablets and smartphones, to Microsoft's Xbox One gaming console and even Internet of Thing devices and servers - Windows 10 is aiming to be the single platform for all your computing needs.

4. Windows 10 will talk to you and you can talk to it

Cortana is Microsoft's answer to Siri and it has been deeply integrated into Windows 10, meaning you will be able to ask you PC whatever you like.

5. Windows 10 means the end of Internet Explorer

While it is still in there somewhere, Microsoft has replaced Internet Explorer with Edge, an all new browser which Microsoft says is going to be "blasting fast" as well as offering features like Cortana and easy sharing and annotation of websites.

6. Windows 10 will be holographic

As we said, Windows 10 will be everywhere and that includes Microsoft's new HoloLens augmented reality headset, meaning developer's apps can easily be tweaked to work in three dimensions.

Microsoft HoloLens
Microsoft HoloLens projects a user interface onto what you can already seeMicrosoft

7. There are seven versions of Windows 10 launching

Windows 10 Home is the main consumer version. Windows 10 Pro will focus on small business. Windows 10 Enterprise is aimed at bigger companies. Windows 10 Mobile is its consumer smartphone play, while Windows 10 Mobile Enterprise is the same for business scenarios. Windows 10 Education is unsurprisingly aimed at schools and universities. Finally, Windows 10 IoT (Internet of Things) is aimed at connected devices.

8. Windows 10 will be a big deal for gamers

From the latest graphics support with DirectX 12 to cross-platform multiplayer with certain Xbox One games, and the ability to stream games from an Xbox One to a PC, Windows 10 promises to be a big upgrade for games.

9. Windows 10 promises developers their app will work on all platforms

To help drive interest in creating apps for the Windows Store, Microsoft is promising developers they only need to write their app once and it will work on everything from desktops to smartphones.

10. The Start Menu is back in Windows 10

One of the biggest gripes people had with Windows 8 was the removal of the Start Menu in desktop mode. Microsoft has clearly been listening and it's back in Windows 10; and it looks great, with the inclusion of live tiles alongside the traditional menu.