I just learned about this neat editor for creating and editing Windows Installer packages (.msi files) and merge modules (.msm files). Orca is just one of many cool installer tools by Microsoft. It provides a graphical interface for validation, highlighting the particular entries where validation errors or warnings occur. This KB255905 article explains more.

Orca is part of the platform SDK and locating the correct download was difficult - a lot of redirected pages and dead ends, but I found it as part of the Vista SDK download as well as the Windows Server 2003 SDK download. Once you install the SDK, look for Orca.msi and install it separately.

Here is a screenshot of running Orca on the Orca.msi file ...