Prompt for master file

Dear list

I can't get AUCTeX to prompt me for a master file when I open a file, 
new or existing. I've tried adding:

(setq-default TeX-master nil)

to my _emacs file, and it makes no difference. I've also gone into the
customize AUCTeX menu and changed (and saved) the value there to 
'query', but it hasn't made a difference. (I know what the expected 
behaviour is, because it always worked for me under Linux.)

best,
Ian

Emacs  : GNU Emacs 21.3.1 (i386-mingw-nt5.1.2600)
  of 2004-03-10 on NYAUMO
Package: AUCTeX 11.55

current state:
==============
(setq
  window-system 'w32
  LaTeX-version "2e"
  TeX-style-path '("style/" "auto/" "/c/emacs-21.3//site-lisp/auctex/style/"
		  "/c/emacs-21.3//var/auctex/")
  TeX-auto-save nil
  TeX-parse-self nil
  TeX-master nil
  )