One question about TeX-command-default

I'm using buffer local variable to define the default LaTeX command as 
something else using "TeX-command-default". It works fine when I use "C-c 
C-c" to activate it. However, if I manually type "LaTeX" once after I use 
"C-c C-c", then thereafter, the default command becomes "LaTeX" 
automatically, it will not change back to the default command I defined. 
Any idea about this problem?

Another strange thing is about preview-latex:

I successfully installed the preview-latex for CVS Emacs obtained from 
http://www.crasseux.com/emacs/.  However, I couldn't install it successfully 
for nqmacs from http://nqmacs.sourceforge.net/ using the same way. My steps 
were:
1. 
/configure --with-tex-dir=/c/texmf --with-lispdir=/c/Emacs/site-lisp/preview-latex
2. make
3. make install
It couldn't make the directory "preview-latex" and copy relavent files there 
for the new Emacs.

BTW: I'm using AUCTeX 11.55
                         MikTeX 2.4
                         CVS Emacs 21.3.50.1
                         Windows XP.
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Best Regards,
Adam