Real Audio did (or maybe still does) provide a means of playing MIDI but it used to be an add-on. If I remember correctly, it relied on software to generate the MIDI sounds and was not very good quality although perhaps better than cheap sound cards with FM systhesis. I am not sure about Winamp but I can not see it trying to take over MIDI.

As for the file assosciation bit, it is a mystery to me. Sometimes the folder options seem to work, other times they don't. I would guess that there is a load of junk in the regitry that overrides some changes.

Jon