The Microsoft link is to a security update, not to the program itself.

Assuming that you don't have the original software on a disk somewhere, you can get a 60-day trial version of Microsoft Digital Image Starter Edition 2006 at (very long url).

Whether this (also discontinued) relative of 'Picture It!' will open your .mix files I don't know (there was some talk of PI and PD versions not always being compatible; might be something to do with layering) but there doesn't appear to be anything else that will do it (the 'insert in a Word doc' wheeze may not work if you don't have the appropriate filters) so you may have nothing to lose.

After this, you won't need me to tell you never to save image files in mickey-mouse proprietary formats. Always use an industry standard format like tif[f] if you want to be able to open them a few years down the line; use jp[e]g only if you don't intend to edit them.