MSIDIFF is freeware to compare or disassemble Windows Installer databases (a "MSI to TEXT" Utilitaire). While it is a command line based tool, the installer creates shell extensions so that the decompile operations can be invoked by right clicking on a file in Explorer.
The output is very easy to read and can be processed by my MAKEMSI tool or can be further processed via my PPWIZARD preprocessor.