Where Does Window's Product Id Get Stored

There are two places at least where ProductId gets stored.

1>  To see the first place, open Registry by going to START-RUN and entering REGEDIT and Navigate to [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion]. In right panel, look for key by the name "ProductId". This is your Windows Product Id.

OR

2> you can navigate to [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion] and still find same field with the name ProductId.