Howdy Kombrig,
Sometimes other programs will stealthily "take over" file associations, or disable them for other programs, which is definitely annoying.
To set the file associations for .mid and .not files manually in Windows, you can do the following:
1. Click the Start button in the lower, left corner on your Windows screen. A popup menu will be displayed.
2. In the pop-up menu, choose Settings / Control Panel.
3. In the Windows Control Panel, choose Folder Options.
4. In Folder Options, click the File Types tab.
5. In the File Types panel, choose "MID" as the "Register File Type". (You can also repeat and choose "NOT" or "KAR" as well.)
6. Under "Register File Type", choose "Notation Composer". If Composer is not offered as one of the registered file types, then set a check mark by "Always use the selected program to open this kind of type", and click the Browse button. In the Browse dialog box, choose \Program Files\Notation\Composer\Composer.exe
After you do this, the whole problem might happen again that the other "selfish" MIDI program will automatically set itself as the default MIDI program if you run that other program again, even once. If this happens, you might want to consider not using the other MIDI (media player) program any more.
ttfn,
Sherry