You should ensure the following path exist in the system registry:
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Windows NT\CurrentVersion\Drivers32

MIDI ports can only be registered there for user-land drivers. Note that on Windows there are only 10 MIDI slots available for registering MIDI ports, they're numbered as MIDI, MIDI1..MIDI9 in that registry path. So it seems that all mentioned slots are used on your system. Try to free some.

Also be aware of the common problem with the Vista and above - that UAC feature. Either running mt32emu_drv_GUI.exe as administrator or turning off the UAC should help.

Ok, in that registry path I have midi0-9 mixer0-9 and wave0-9... and the value of each of them is wdmaud.drv. I assume that means that all 10 MIDI slots that Windows provides are being spammed by the default midi driver? Can I just modify or delete one in order to allow the mt32 registry value to write? Thanks for the reply; it seems to be spot on.
Oh, and UAC is terminated on my machine! I couldn't stand its incessant whining.

Unfortunately, I can't say for sure whether it's normal behavior for you soundcard driver or traces of a malware... Those could be a trace of some kernel MIDI driver as well. At least on my system only 1 port allocated by default - MIDI - for wdmaud.drv, and that internal M$ dmusic synth still works if I remove this entry.

And yes, UAC is really annoying thing... 😀