First thing to keep in mind is that the uninstall is controlled by the setup program, in this case SETUP.EXE, and not the RunDLL32 executable.  There are no generic command line additions you can make to change how RunDLL32 is handling the execution, as it really is doing nothing more launching the CTOR.DLL and passing it command line parameters, which in this case are  both the name of the actual uninstall program (SETUP.EXE) and additional parameters for the uninstall program to use. 

My second guess would be that you want to uninstall this from more than one workstation in an automated fashion.  That's good as might take a bit of work to change the command line you have into one that can be called silently.  The first place I would start is with the vendor's web support area to see if they provide uninstall instructions beyond what you already have.  Very often the uninstall command line placed in the registry works well for a manual removal, but is not meant for an automated removal, which is often done by administrators when they need to clean software off of workstations, or by a higher version of the same software that needs to first uninstall the old program then install the new.  If the vendor has no additional information, I would next look at the documentation for the uninstaller itself.  Based on information contained in your command line (which we snipped for readability), InstallSheild provided the installer used.  Look for information on setup command line parameters and options to see if adding a generic switch to the uninstall command line is all that is required to remove it quietly, or if you have to take further steps of recording answer files.  The latter can be as easy as adding a -r to the end of your uninstall command line, performing the uninstall manually, then taking the recorded setup.iss file, renaming it to uninstall.iss, and then using the -f1 switch to specify the uninstall script of subsequent uninstalls.

You might also try resources such as www.AppDeploy.com to see if anyone else has already completed a silent uninstall for your product and has posted the answer.

The word of caution I have for this style of external device is that Serial and Parallel ports are going the way of floppy disk drives and PS/2 mouse and keyboard ports (extinct).  Most external devices can be used long after the primary unit you bought it for has been replaced, so if you purchase a device that is not the most compatible device available, it will cost you more down the line if you need to replace it.  My advice then is that if your laptop does not have the FireWire port already, invest in a PCMCIA FireWire card purchase an external DVD burner that can use FireWire.