John:
Until someone more qualified stops by, I'll try and help. I use MIDI quite often, so maybe I can recall some things I have tried in the past.
You reference that you USED to be able play MIDI files okay. It would help to know what occurred when things worked: what program played the mid files before?
Did you install new software between the time it worked and stopped? For instance, did you download a new midi player? Or maybe you visited a site that had a midi player on site, or downloaded from that site?
Which program would you like to use to listen to MIDI? You know John, there are so many programs out there. With Real Player, Windows Media, Quick Time it's easy to get the preference directions twisted up.
Those programs have prompts on install that ask you what files you want to associate to the program. It sounds like you have one program that is designated to run the midi files, and another program that is trying to play them, but can't.
Open all your MIDI players one at a time and check the preference tools to see what files are assigned to play. See if you can play the default demos on the players. I'd start with the program that runs the .wav files. Let me know what it is: if I have it I will dig around a little more and give more specific directions.
Something changed your preference settings, imo, so it would be helpful, again, to revisit any changes you made to the computer since the .mid files worked properly.
Peabody