GetMSIProductCode comes in two little programs. Unzip them into a directory of Your choice.
You can choose between two ways to use it:
GetMSIProductCode.exe needs no installation. It is used by dragging a msi-file on it or started with a doubleclick.
When You drag a msi-file on it, it simply shows the produtc-code and puts it into the clipboard: 

When You start it, it shows a window, that tells You to drag a msi-file on it: 

After dragging a msi-file ont this window it shows the product-code and put it into the clipbpard.
GetMSIProductCodeAndQuit.exe installs iteself into the explorer´s context-menu when You start it once 
as administrator.
It shows a confirmation-message: 

When You now click right on a msi-file, the contex-menu shows a new entry ´Get ProductCode´: 

When choosen, it shows a little confirmation-message, which disappears after two seconds, that the product-code was put into the clipboard: 
 
Uninstall GetMSIProductCodeAndQuit
To remove the context-menu for GetMSIProductCodeAndQuit, start 
 GetMSIProductCodeAndQuit.exe  with the argument 
/unregister.
You don´t need to remember this command, as You can start 
 GetMSIProductCodeAndQuit.exe  with the argument 
/? to get help: 
