Windows Installer uses the ProductCode to identify an application during subsequent installations or upgrades; no two applications can have the same ProductCode. To ensure a unique ProductCode, you should never manually edit the GUID, instead you should use the GUID generation facilities in the Product Code dialog box.

The Product Code and Package Code are different properties. Visual Studio does not support setting the Package Code. However, you can access and modify it by using the summary information screen in Orca.exe, a table editor for creating and editing Windows Installer packages and merge modules. See Orca.exe on the MSDN Web site.