I've lost track of which of the multitude of "music and multimedia" programs are the offenders and which are the nice guys; but I believe that RealPlayer is one that "popped up" from a preinstalled (unactivated) "bonus software" plant on my brand new computer.
IT, or one like it, presented a lengthy RANT about how unscrupulous other media programs were - because they sometimes replaced IT as the default program, and explained that IT included a "protection" program that would reset IT as the default if some other program "took over" while it was turned off.
In other words, if you ever installed IT, the IT would prevent you from ever installing and using any other similar program. This was supposedly to "protect" you from all of those inferior and ill-mannered programs that you might be tempted try to use.
If you did not KILL ITs reset VIRUS when you uninstalled IT, the reset VIRUS may still be lurking there, resetting your file associations to IT, even though IT isn't there to work. Quite likely, ITs reset VIRUS command would be buried somewhere in Windows System folder, and/or in Start and/or Startup folders - remote from where the main program files were, and easily missed during an "uninstall."
First step would be to check "file associations" and see if the program you want is listed to open the file types that are giving you problems.
Next step would be to change the file associations to what you want.
Third step would be to see if the settings you entered stay put after you've tried to use the file types involved.
The last step, if needed can get pretty messy, but depends on your confidence in "treading in dangerous places" and on the specific operating system version you're working with.