HTG Explains: How Does Windows Activation Work?
 

Windows Activation, introduced in Windows XP, checks in with Microsoft when you install Windows or get a new Windows PC. This is an anti-piracy feature — it’s designed to annoy you if you’re using a non-genuine copy of Windows.

Luckily, Windows Activation has been toned down after its introduction in Windows XP. Even if your PC fails activation, it will continue to function until you can fix the problem — unless you’re using Windows XP. If you are, you should upgrade anyway.

http://www.howtogeek.com/182847/htg-...tivation-work/