The easiest InstallShield answer involves using prerequisites, as they support conditional installation, downloads, etc. Unfortunately creating prerequisites is only supported in the Professional tier and higher, not in the Express tier. Most other clean ways you could approach this will be equally hard in or out of InstallShield as they will require custom code. Less clean approaches may allow you to include these files as Setup Files and launch them from custom actions - the MSI would have to be launched from a dialog, but the exe may be possible at a different time.
