MSI actions, such as self repairs, are logged in the application event log. In the details of the log entry for the error, you may get lucky enough for it to call out the problem file or registry entry by name. More often you will see a ID identifying a specific component. This is a Component GUID. To identify the component, you will need to open the MSI in your favorite Windows Installer Editor and perform a search for this ID.
From here, the fix will depend upon what you discover the problem item to be. The most simple solution is to remove the item and cross your fingers!