Also recommend FTDI based devices for your best chance and keeping the COM port low. If possible even down to COM1-4. FTDI is easily changeable but so are many others. Sometimes you have to juggle things around though since the lower ports are occupied by Bluetooth devices among other things.

If the software is so old and primitive and is hitting the hardware directly in a "DOS" window, you might have bigger problems but I seem to remember experiencing Windows properly intercepting that somehow.