Windows 10 Windows 10 Activation explained..

[kemical]

Recently the process of Windows 10 activation and what happens in terms of activation keys has been a little vague. We now understand that Microsoft has changed the activation process significantly so much so that the need for actual keys has almost been done away with.

If one now installs Windows 10 (this also applies to Windows 7 and 8.1 too) on a new device you need only input the activation key once. Any installs after that whether they be a clean install or not won't need an activation key. This is because your status is stored on an 'Activation Database'.

Many users who upgraded from Windows 7 or 8.1 will notice their Windows 10 is also activated but no mention of a key.
What happens when you upgrade to Windows 10 is this:

As long as your current install whether it be Windows 7 or Windows 8.1 is genuine and activated then the information regarding the activation will be stored online on the Activation Database.

When you upgrade to Windows 10 the set up will check online about your status and as long as your previous windows is genuine as well as activated a Windows 10 licence is issued. This will also be stored online with your other activation details. Microsoft calls this 'Digital Entitlement'.

Once this information has been stored one can then go right ahead and perform a clean install without having to input an activation key as of course it's stored online. Just keep pressing 'skip' when asked to input a key.

So if you want a free copy of Windows 10 then one must upgrade first of all simply because of the process described above and of course have a genuine activated copy of either windows 7 or 8.1.

One can of course simply buy a copy but once again when installing the key only needs to typed in on the first install.

Further reading can be found here:
Microsoft quietly rewrites its activation rules for Windows 10 | ZDNet

Today Microsoft announced that users can use their old keys to activate windows 10:

We have received a lot of feedback from Insiders on making it easier to activate Windows 10 on devices that take advantage of the free upgrade offer to genuine Windows by using existing Windows 7, Windows 8 or Windows 8.1 product keys. If you install this build of the Windows 10 Insider Preview on a PC and it doesn't automatically activate, you can enter the product key from Windows 7, Windows 8 or Windows 8.1 used to activate the prior Windows version on the same device to activate Windows 10 by going to Settings -> Update & security -> Activation and selecting Change Product Key. If you do a clean install of Windows 10 by booting off the media, you can also enter the product key from prior Windows versions on qualifying devices during setup.

Reference

 

[kemical]

Some users have also wondered if Windows 10 can be transfered to another machine like a new computer especially if the licence they upgraded from was full retail.

Unfortunately the answer is no.

The upgrade is tied to the machine you upgraded on.

If you know your going to upgrade your computer soon then don't upgrade to windows 10 yet. The upgrade offer is open for a year so you have time yet.
Wait until you get the new machine, pop your full retail copy of either win 7 or 8.1 on and then upgrade to Windows 10.

Once upgraded as outlined above the install will then be tied to that machine and that machine only.

 

[kemical]

Something I'd missed, thanks to Neemobeer for the info.

Starting with the November update, Windows 10 (Version 1511) can be activated using some Windows 7, Windows 8and Windows 8.1 product keys. For more info, see the section Activating Windows 10 (Version 1511 or higher) using a Windows 7, Windows 8 or Windows 8.1 product key in this topic.

Activating Windows 10 (Version 1511 or higher) using a Windows 7, Windows 8 or Windows 8.1 product key
During the free upgrade, you can use a valid Windows 7, Windows 8 or Windows 8.1 product key to activate Windows 10(Version 1511 or higher). The following types of product keys are supported:

• Product keys included in the packaging or the Certificate of Authenticity (COA) attached to your PC that came installed with Windows.
  
  
• Product keys included in the confirmation email that you received after buying Windows from an online retailer.
  
  
• Product keys included in the box that the Windows DVD came in.
  
  
• Product keys that you bought using Windows Anytime Upgrade or Get Genuine Windows.
  
  
• If you bought a Windows 8.1 Pro Pack product key, you can use it to activate Windows 10 Pro.
  

Note

• Some Windows 7, Windows 8 and Windows 8.1product keys can’t be used to activate Windows 10, such as product keys for volume licensing, including Generic Volume Licensing Keys (GVLK) and Multiple Activation Keys (MAK), and Windows 7 Enterprise, Windows 8 Enterprise and Windows 8.1 Enterprise.

Reference