Where does Window's Product Id 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\WindowsNT\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.