The product code is a Globally Unique Identifier (GUID) that is the principal identification for an application or product. GUIDs are 128-bit values that are generated using an algorithm defined by the Open Systems Foundation to guarantee uniqueness across time and space.
When to Change the Product Code
• The original and updated product installations will be installed on the same machine.
• The name of the .MSI file has changed.
• The component code for an existing component has changed.
• A component has been added or removed from an existing feature.
• An existing feature has become the child of an existing feature.
• An existing child feature has been removed from its parent feature.
When not to change
•For minor changes to a product, the product code does not need to be changed.
•When you add a new child feature to an existing feature that has entirely new components.
For details on how a WIndows Installer GUID gets generated, refer this link http://www.msigeek.com/282/windows-installer-guids
Microsoft MVP [Setup-Deploy]