How do I determine the directory that the .msi was launched from?
Use the predefined Windows Installer property SourceDir. This properrty contains the source path of the .msi that is currently running. This property is useful when you need to reference the source location of the .msi you are running. This property is case sensitive.
For example, you may want to execute an .exe from your .msi. However, due to the size of the .exe, you do not want to embed it inside the .msi. Instead, you distribute the .exe in the same directory as the .msi. In this case, you can call an Execute Program from destination custom action to run the external .exe and reference the SourceDir property.