If you did fiddle with the wiring, I hope you did it right
Usually if you can't select options in a driver, it's because hardware isn't installed properly. You said you had another machine with a built-in com port. Since it's more likely that the com port is operational on that machine (check that it's enabled in the BIOS), try it on that machine, but completely uninstall the Unitor drivers via add/remove (or if that doesn't work, use the procedure in my last post) and reboot before you do.
BTW, if the COM port was disabled in the BIOS, reboot once for the COM drivers to take and then shutdown and connect the Unitor before you restart and then follow the XP installation instructions.
On your original target machine, I still think your most likely problem is the COM port drivers. Try to get XP to find new drivers online. Or maybe write to Dell and ask them that you need a COM port and ask them for a recommended card. That might be the most direct way of making sure there isn't a hidden resident COM port in the machine, and/or getting a card that works for it.
Of course, nothing has ruled out the possibility that the Unitor serial connection is dead... but we remain optimistic
