If you are one of those who got official windows 10 update from any previous version of windows such as win 7 or 8 and unforunately your laptop or dektop crashed and you need to reinstall the OS. You will be in doubt weather you will get the activated origina version of the win 10 OS.You need to follow these steps..
1. Download the win 10 image file from microsoft website.
2. Install it in your laptop or desktop.
3. Skip the step if it asks for the activation key for your variant of windows say home edition or premium etc.
4. Let the installation get complete and restart your PC.
5. After logging in go check your windows properties you will find that it will be already activated.
The reason of how it happens is simple, microsoft stores your PC information and binds it with the official installation key, so that when you reinstall it automatically it follows a search alogorithm and pulls the activation key to your PC.
Note- This works only if you own an Original windows version.Hope this information helps some one.