You can't use Program Files\ in vista because of file virtualization. Vista cannot write to anything inside program files, virtualizing the written data to a safe place outside program files instead. It's not *always* read back in the same way it's written out, so mIRC can easily "not see" the mirc.ini it wrote last time and try to write a fresh one.
Long story short: This is the reason AppData is now the default behaviour. Don't use Program Files\, it will just make your life more complicated.
