Where does Window's ProductId get stored ?

There are two places at least where ProductId gets stored. 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 pane, look for key by the name "ProductId".
This is your Windows Product Id. Alternatively you can navigate to
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion] and still
find same field with the name ProductId.