I have never heard the term "longhorn" loader before. Nor "supergrub". But the mbr can only point to one place. If it is pointing to grub then that is all it points to. It sounds like you use grub to chainloader into your longhorn loader. If the mbr is trashed and you restore it, you are back to normal. I like to build a little floppy that will invoke grub. Then if someone scribbles on my mbr, I just boot from the floppy, get a grub command line, and fix the mbr.