The following, taken from the emacs faq, doesn't work:
(add-hook 'emacs-startup-hook
               (lambda () (w32-send-sys-command ?\xF030)))

I ran emacs with only this code in the init file. It appears to maximize
the window then resizes it again.


But the following does work:
(add-hook 'emacs-startup-hook 'toggle-frame-maximized)

This is on windows 8 with version
*GNU Emacs 24.4.1 (i686-pc-mingw32)*
* of 2014-10-24 on LEG570*

I also tried window-setup-hook. Didn't work. Note that the code that uses
w32-send-sys-command works fine if i evaluate it with C-x C-e.