little problem (avoid automatic creation directory .emacs.d)

Dear all,
I am a newbie Emacs user.
I try to install my super editor :-) into USB and I use the 24.5 version.
To complete the procedure I customize its behaviour.  I write the following code :

"(defvar usb-drive-letter (substring data-directory 0 3)) 
(defvar usb-home-dir (concat usb-drive-letter "home/"))
(setenv "HOME" usb-home-dir)"

in site-start.el which is included in...share\..\site-lisp and I add in my \home directory , \.emacs.d\init.el file.
The little problem is that:
when I use the editor in windows 7, it  creates also the folder \.emacs.d\  within ..\AppData\Roaming\
but this path is not used (obviously because root is my usb home and the config/user folder is found)
Otherwise when I just run Emacs under windows XP I received in buffer-window a message:
"warning (inizialization): unable to create 'user-emacs-directory' (~/.emacs.d/) ...." .
Just later the editor continue in correct mode with the exact init file.
I suppose that it have a default directory value (AppData\roaming) which is not found at the start process  (initialization phase).
As a consequence the editor writes the warning in buffer. After it overwrites the folder in my $HOME$ that it just exists and it contains the configuration file.
What can I do to stop the buffer-warning-message in win XP and to avoid the writing (useless) process in AppData\Roaming in win 7?
I thank you to all in adavance for your precious help
With my best regards
Alessandro