When you buy a copy of Windows 7 in a shrink-wrapped box, you’re allowed to install it on one — and only one — PC. To ensure that you stick to that guideline, you’re required to activate Windows 7 within 30 days of installation.
You can also begin the activation process by clicking the Activate Now pop-up in the notification area of the taskbar. The easiest way for you to activate is online.
After you send your activation key, the Windows 7 Product Activation program examines your PC and then mixes up the serial numbers with the product key to come up with a unique 50 characters installation ID. It then checks with Microsoft to see if anyone else has tried to activate that same copy of Windows 7.