how do I find my windows 10 product key before reinstalling

If the PC came with Windows, the key is stored in the firmware. Open PowerShell and run: (Get-CimInstance -Query 'select * from SoftwareLicensingService').OA3xOriginalProductKey If that comes back blank, you've most likely got a digital licence tied to the hardware, and a clean install of the same edition will just reactivate on its own once it's online. Worth linking the licence to your Microsoft account under Settings > Update & Security > Activation before you wipe, just as insurance.