I’ve still got the original keys for win 3.1, 95, 98, 98 SE etc. -- *Somewhere*. I’ve also bought more than one key at a time, e.g. when Microsoft was selling win8 pro upgrade keys cheap [$35]. In the past I’ve found it easier to use key finder apps than look through our records, &/or for figuring out which key went to which device [something I’m sure I’ve got recorded, but I’d still have to look for it]. I can’t see a reason to install & use Windows Product Key Finder Personal Edition however, when Nirsoft Produkey is free, portable, and tiny. nirsoft[.]net/utils/product_cd_key_viewer.html
Not all software registration or activation keys are stored in the registry, and of those that are, it may not be in a format for an app like Windows Product Key Finder Personal 2019 or Nirsoft produkey to find & decode [keys are often stored encrypted]. Also note that with win10 tied to the hardware, based on component hardware IDs, I’ve seen Windows keys for activated copies of win10 that do not match what was originally input on activation.
That said, with win10 tied to the hardware, once it’s activated you often don’t need the key any longer, though it’s probably a good idea to save it somewhere, e.g. if you have to contact Microsoft support because something went wrong & a future copy of win10 doesn’t activate. Associating your win10 license with a Microsoft account may help with that sort of thing, allegedly anyway, but I’ve never had to try it. In the past when you upgraded hardware, e.g. built a new PC or upgraded the motherboard & CPU, you needed the original key to re-activate Windows 7 or 8. While it’s possible to buy a license for Windows 10 that will work on new hardware, the licenses you got from upgrading win7 or 8, and those that come with new hardware, will not work on another device.
To find out what version of win10 you have, e.g. OEM, retail etc., run the command prompt as admin & use slmgr /dli . Depending on the type of license you have, you **may** be able to use your win10 key on another device if you deactivate it on the old one, e.g. before you upgrade the motherboard & CPU, or if you want to use a Pro key on a device that came with win10 Home, using the command prompt running as admin with slmgr /upk . I’ve read online that deactivating the key on your old hardware is both necessary & unnecessary. If the activation won’t work, you can try win10’s Get Help app. To remove the key that’s stored in the registry without effecting activation, so an app like Produkey won’t find it, run the command prompt as admin and use slmgr /cpky. In win10, to run the command prompt as admin, you *may* be able to right click the Start Button & select it from the menu – depending on how the switch is set in Settings, that menu will show the Command Prompt or PowerShell. Otherwise it’s under Windows System in the Start Menu – right click Command Prompt -> More -> Run as administrator.
Windows Product Key Finder Personal Edition isn’t a terrible installation, though relative to the portable Produkey at 129 KB, it might seem terribly inefficient – the 32-bit version of Produkey is only 81 KB (!). You get the program’s folder, holding 8 files, 2 folders, ~7 MB, plus a C:\ProgramData\ Caphyon\ folder, plus a C:\Users\ [UserName]\ AppData\ Roaming\ XenArmor\ folder, plus an a2126.msi file added to C:\Windows\ Installer\, along with a file named “SourceHash{FF59EE1A-B8A5-4D5A-BA41-6FB981CFC977}”, and finally a C:\Windows\ Installer\ {FF59EE1A-B8A5-4D5A-BA41-6FB981CFC977}\ folder storing a couple of files including GooglePasswordDecryptor.exe. The registry gets ~270 new entries spread across too many keys to easily list here.