Are you sure they are not MVP files? I've been digging around the Net for some info on this "MVY" extension and it seems that the search engine wants to default to the "MVP" extension which belongs to Movie Edit Pro. In any event, Vista had, and to date, continues to have some very serious issues. Check your Preference Options and see if you have an option to save to another video extension.
By the way, you may want to consider dumping Vista for (at least) Windows 7. Much more stable.