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Activation helps verify that your copy of Windows is genuine and that it has not been used on more computers than the Microsoft Software License Terms allow. In this way, activation helps prevent software counterfeiting. With an activated copy of Windows, you’ll be able to use every feature of Windows.

Is activation the same as registration?


No. Activation is required, but registration is not. Activation is the process of ensuring that your copy of Windows is used according to the Microsoft Software License Terms. Registration is the process of entering information (such as your e‑mail address) to sign up for product support, tools and tips, and other product benefits.
Note If you install the same copy of Windows on a second computer and use the same product key that you used to activate Windows on the first computer, you can no longer use this copy of Windows on the first computer. To continue using Windows on both computers, you must buy a new product key to activate the second installation.

EULA:

1.1 Installation and use. You may install, use, access, display and run one copy of the Software on a single computer, such as a workstation, terminal or other device (“Workstation Computer”). The Software may not be used by more than two (2) processors at any one time on any single Workstation Computer.