I don't have the setup to build emacs on Windows. So I tried applying your
patch just to <emacs extracted dir>/share/emacs/25.0.93/lisp/faces.el,
recompiling faces.elc and restarting emacs. But the variable-pitch is still
"Sans Serif" family instead of being set to Arial.

On my windows, C-h v system-type gives windows-nt. So I replaced w32 with
windows-nt, recompiled faces.elc, restarted emacs, and still nothing
happened.

I am not sure I any of my previous emails were received, but FWIW, I cannot
figure out if edits to that faces.el are being effective on Windows, or if
that patch is working for me.

On Tue, Apr 26, 2016 at 2:30 PM Eli Zaretskii <eliz@gnu.org> wrote:

> > Date: Tue, 26 Apr 2016 20:26:41 +0300
> > From: Eli Zaretskii <eliz@gnu.org>
> > Cc: chriszheng99@gmail.com, emacs-devel@gnu.org
> >
> > I cannot just revert that commit, because it fixed a problem that is
> > no less serious.  Ideas welcome.
>
> Could people who are affected please try this kludgey workaround, and
> see if it gives good results?  Thanks.
>
> (Of course, if someone has got a better idea, please speak up.)
>
> diff --git a/lisp/faces.el b/lisp/faces.el
> index 612bd16..3d3adaa 100644
> --- a/lisp/faces.el
> +++ b/lisp/faces.el
> @@ -2281,7 +2281,9 @@ fixed-pitch
>    :group 'basic-faces)
>
>  (defface variable-pitch
> -  '((t :family "Sans Serif"))
> +  '((((type w32))
> +     :font
> "-outline-Arial-normal-normal-normal-sans-*-*-*-*-p-*-iso8859-1")
> +    (t :family "Sans Serif"))
>    "The basic variable-pitch face."
>    :group 'basic-faces)
>
>
> --

-- 
Kaushal Modi