Filetypes keep changing from their default?

A filetype on my computer was recognized by my computer for quite some time... After I installed the program that it was associated with, that program became it's default, and the icon that the program had in store for the filetype was set, and the program was set as it's default program.

A little while ago this changed constantly, the filetype was no longer recognized by my computer, and I had to set what program it was associated with every time, so that my computer could comprehend the file.

It keeps changing from what I set it as, to unrecognizeable. Constantly... Without me even doing anything even closely associated with the files or the filetypes.

Any help?