Here's what you need to know about what will happen to your free version of Windows 10 if you need to reinstall or change your PC.

Owners of Windows 7 and 8.1 will be able to upgrade to Windows 10 for free but can they keep using that copy of Windows 10 if they need to reinstall Windows or replace their PC?

Reinstalling an upgraded version of Windows 10 on the same machine will be possible without having to buy a new copy of Windows, according to Microsoft.

People who have upgraded to Windows 10 will be able to download media that can be used to clean install Windows 10 from USB or DVD. There will be no need to purchase a new copy of Windows 10 provided it is being installed on the same Windows 7 or 8.1 machine that was upgraded to Windows 10. The installed version of Windows 10, Home or Pro, will be the same as the version they upgraded to.

When installing Windows 10, users will be able to skip the requirement to enter a product key and Windows 10 will activate automatically online.

TechRepublic's sister site ZDNet also recently published a guide for people running early builds of Windows 10 under the Windows Insider Program, detailing how to create ISO files from the OS that can be used for a clean install.

However, people running an upgraded version of Windows 10 may need to buy a new copy of Windows 10 if they switch to a new machine or "significantly upgrade the device" - which Microsoft counts as replacing the motherboard.

Ian Moulster, Windows product manager, said: "My understanding is it's licensed to the device and not to you, so it's specific to the device and you wouldn't be able to use it elsewhere."

A post on answers.microsoft.com suggests that only those who have upgraded from OEM versions of Windows 7 and 8 will need to buy a new Windows 10 license after replacing a computer's motherboard. Those upgrading from a retail version of Windows 7 or 8, the post says, would be able to transfer their copy of Windows 10 to a new machine or PC with a new motherboard.

Most owners of Windows 7, 8 and 8.1 operating systems will see a Windows icon in their taskbar that will allow them to "reserve" their free upgrade to Windows 10. The 3GB file can be downloaded from 29th July. The free upgrade will be available until July next year and those choosing to switch to Windows 10 can cancel their reservations at "at any time", according to Microsoft.

To upgrade using Windows Update users must be running Windows 7 (Service Pack 1) or Windows 8 (Windows 8.1 Update). Other Windows 7 and Windows 8 users will still be able to upgrade to Windows 10 by downloading an ISO image from Microsoft.

The five million Windows Insiders who have been testing Windows 10 will also get Windows 10 on July 29.

Windows 10 Enterprise and Windows 10 Education, editions will be available from Saturday, August 1. Neither version is available under the first-year-free promotion and both will be available for download from Microsoft's Volume Licensing Center.

More on Windows 10