It will reactivate automatically on reinstall without entering a key (if you are reinstalling on the same machine you activated Windows on).
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If you upgraded to Windows 10 for free the license is tied to the computer hardware or your Microsoft account.It will be read automatically on reinstall if your computer is from a manufacturer such as Dell or HP etc. Windows 8.1 or 10 users will either have a separate key if Windows was purchased as a standalone product or the key will be embedded in the system BIOS. Windows 7 users will have a product key sticker.
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It’s also a good idea to clean install Windows 10 after upgrading so you are not transferring problems from your old Windows install. Simply using Windows over time will eventually degrade its performance. It doesn’t really matter what version of Windows you have, the best way to get your system working at its full speed and as issue free as possible is with a clean install of the operating system.