In MC "Options\File Types" make sure that the file types you want to be opened with other programs have the Playback Method set to Automatic.
Then in Windows, go to "Control Panel\Programs\Default Programs\Set Associations" and change the default program for each file type.
You did tell MC to "Hijack" all your image and video file if you just ran an Express MC installation, because that type of installation does assign MC to lots of file types. If you had done a Custom installation you could have controlled which file types MC would open by default.
While you are thinking about this, in MC go into Tools\Import\Configure Auto-Import, select each folder location you are watching, click Edit, and check which file types MC will import for each of those locations. If MC is importing files into its database that you don't want it to, that is the place to fix the problem.
BTW, transferring your Playlist from MC19 to MC21 should have been as easy as making a backup of your MC19 library, installing MC21, and restoring the MC19 backup into MC21. Done. If you had asked someone may have saved you a lot of time and angst. Or of course, you could have search the Forum and/or Wiki, as there is lots of good information in both.