Windows 10: Where to buy product key for already installed un-activated OS

I originally bought a PC pre-built, I had a product key with the computer specifically for Windows 10 Pro, but then sold the motherboard of this prebuilt computer to a person I do not talk with anymore and reinstalled windows onto a new M.2 I put in my almost completely modified computer. I still have the product key, it's on a sticker stuck to the side of the case of my old computer.I have given up trying to use the key because windows keeps giving me an error whenever I try to use it.I want to buy a product key for the Windows 10 Pro I

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Activate windows 10 after re-installing (I have lost my product key)

I own a Lenovo T430 laptop. In 2017, I installed Windows 10 using my original Windows 7 product key. Unfortunately, I cannot find that product key anymore. I need to re-install windows 10. Will it be automatically activated after the installation, if I skip the step of asking the product key during the installation?

In my opinion, I believe that my laptop will have a digital entitlement stored on the Microsoft Activation Servers and this will not be a problem. Is that true?

If not, please suggest me a solution or a workaround.

 

Windows 10 product keys and activation - and the KMS angle

Changing the product key to the generic Windows 10 product key will reset activation to the standard retail channel for activation. It might just activate (if there was a previous digital license for Windows 10 for that computer established with Microsoft). Once Windows 10 is back on the retail activation channel, then more than likely changing the product key to the Windows 8 product key using the same method may work.

 

Windows 10 product keys and activation - and the KMS angle

I raised the same queries in the Seven Forum where I have long been a member and (thanks, Bree...) have had some useful advice over there. I also posted here as I realised I was making a transition, and maybe the Win10 experts would be able to assist.

I think the quote above describes almost exactly the situation with my (second hand, formerly corporate) ThinkPad.

It now emerges that my ThinkPad originally had Win 8 Pro (RTM...?) as OEM; probably had that replaced by a corporate OS image and/or Win 10 Pro.... hence the KMS activation problem; and according to Bree, I could now do a clean install of Win10 which would find and be activated by the embedded Win 8 key.

If that's so, then I have a clear route to follow. Just need a step-by-step set of instructions for that clean install. Is there a tutorial, and/or any pitfalls to be aware of?