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#11
All installs of Windows 10 will have a product key, if you don't supply one when asked during the install then Setup will install a generic key, one that allows Windows to be installed but won't activate it. Each edition has its own generic key, listed here:
Generic Product Keys to Install Windows 10 Editions
How old is this PC? What version of Windows was it supplied with originally? For OEM pre-installed Windows 8 or later the key will have been embedded in the bios firmware by the OEM, and can be read with ShowKeyPlus
For a Windows 7 PC a key can be found on the CoA sticker, either on the underside of a laptop, inside its battery compartment or somewhere on the side or back of a desktop PC.
Any windows 7, 8, 8.1 or 10 key you may find in this way can be used to activate an already installed unactivated Windows, just change the installed key to the one you have found.
NB: a Home key won't activate Pro. A Pro key can be used on an installed Windows 10 Home, it will upgrade it from Home to Pro and activate it as Pro.
For a Win8/10 key found by ShowKeyPlus it will tell you the edition of the key (if it says Core that means Home). For a Win7 key ShowKeyPlus has a Check Edition option where you can type in the key and it tells you what it is.