Not working for me, morbid.
hm. maybe mirc overwrite this ini-key when exit. just check [about] and [user] section in your $mircini if you install mirc in portable mode, or HKEY_LOCAL_MACHINE\SOFTWARE\mIRC\LastRun in registry editor (Start -> Run -> regedit).
Or you can also just go to start then Run > Regedit > current_user > Software > Delete the mirc folder.