I am trying to set the default directory to
(setq default-directory "D:/")
but when I type
c-x c-f, it is showing
D:\software\emacs\bin, which is the installation directory.
The issue is indeed likely the same as the one in Setting default-directory does not change find-file directory but here's a "solution" for your specific case:
C-x C-f shows you say
D:/software/emacs/bin/ and you want to open, say,
D:/Documents/mynotes.org, then just type something like
/d/mynot TAB RET and you should be done. This is because
// in this kind of file prompt is treated to mean "ignore everything before the
//". Just typing
/ should already make this clear by coloring the
/software/emacs/bin/ in grey.