Solution to setting ModPlug as the opening application:

To prevent your system from "losing" ModPlug as a valid application, when upgrading to a new version of MPT, rename the old folder (but remember the current folder name), and name the new folder (where your mpt executable will be) the same name as the old folder was. Alternatively move the applicable files from the old MPT build somewhere else, and move the new build into the current folder. Make sure the MPT executable's name AND PATH are the same as previous. The object here is to keep all the new files the same as the old (registered) files with the same name and the same path.

If you move things around, Windows loses sight of the valid "registered" executable, and can't find it to make it available to the Windows contextual menu (shell) for opening files. When you install an application, Windows sets up its validity by putting it in its registry, including the file path and its name. When you move the application or rename, the registry key is no longer applicable.

If ModPlug is not showing up as an opening executable (an application that can open files that have certain extensions), here's how you can fix it:

1. Either A. click Start, click Run, type %systemroot%\syswow64\regedit, and click OK; or B. open the WINDOWS folder in your main drive (usually C:/WINDOWS) and open the "regedit" executable.

2. On the left side is the directory pane. Open the "HKEY_CLASSES_ROOT" directory (it may be open to it by default already), and scroll down to the Applications folder. This holds the registry keys for all valid applications on your system.

3. Look for the correct name of the current ModPlug executable, such as "mptrack.exe", and open this directory.

4. There should be only one subdirectory "shell." This is the marker for Windows to include this application in your opening executables list. Open this directory, and its subdirectory "open" and ITS subdirectory "command."

5. On the left side of the RegEdit window, you'lll see the Open registry key. The pathname is what you'll need to correct. Double-click the "(Default)" icon, and enter the correct pathname.

6. To verify that what you did was correct, find a track file for MPT to open. Right-click on it to show the contextual menu, and choose "Open With..." The Open With dialog will appear and if you scroll down you should see the current version of ModPlug (or "Open MPT") in the list. If so, you've now corrected the registry.