MsiDiff

MSIDIFF is freeware to compare or disassemble Windows Installer databases (a "MSI to TEXT" utility).

The output can also be understood by my MAKEMSI tool (it is an easy way to produce MAKEMSI scripts).

While MSIDIFF is a command line based tool, the installer creates shell extensions so that the decompile operations can also be invoked by right clicking on a file in Explorer.