Any program can 'tell' the OS that it is capable of opening 'such and such' a type of data file. Presumably the likes of Maxim and CCDStack do just that. Photoshop cannot open FITS files, at least not 'natively' - it requires something like FITS Liberator to be able to do this, whereupon it will also 'tell' the OS that it can now open FITS files.
I am not sure what your problem is - I don't ever remember my Vista64 OS 'not' using PixInsight as the default program for opening FITS files and, if there was such a time, I am sure that I just 'advised' the OS which program (i.e. PI) that I wanted to be the default to be used.
Then again, because PI is 'always running' on my machine, I may well open FITS files more often by 'drag-n-dropping' via the TaskBar. Each to their own, I suppose.