I found an answer a year-or-so back that explains you have to manipulate the registry to do things right:
To start Emacs maximized put this line
at the end of your ~/.emacs file:
(w32-send-sys-command 61488)
If you don’t want the Emacs tool bar
you can add the line (tool-bar-mode -1) [NOTE: value is 0 on original page]
to your ~/.emacs file but Emacs won’t
fully maximize in this case – the real
estate occupied by the tool bar is
lost. You have to disable the tool bar
in the registry to get it back:
[HKEY_LOCAL_MACHINE\SOFTWARE\GNU\Emacs\Emacs.Toolbar]
@="0"
If you look in the EmacsWiki under W32SendSys command-codes you’ll find that 61488 is maximize current frame