Hmmm. This may very well all have been because my XP is unstable, but steeladept came the closest without being near:
open Explorer .. and .. go to Tools>Folder Options>File Types and browse down to your file extension (.avi for example). .. click on the file extention and in the box below, click on the "Change" button by "Opens With:" .. Now just browse to the application executable that you want to open that file type with and it is associated.
- because, I did all that, which steeladept said, only: it didn't help!
But then I clicked
Reset, and wow!! All file types were changed back to the right types again!
Edit: it would have been a problem if I didn't first changed some file type, because then there would have been no "reset" button! So I actually had to do it this "wrong" way, in order to be able to do it the right way! Now
that doesn't sound too sane, does it.
The correct answer may be unknown, but the problem is solved, so who cares!
Thanks a lot, all of you!