how to run shell like app before explorer.exe?

[DVirus]

hi..
I have a program which is controlling system LOGIN
for all users.. It's executing from regsitry/Run
section now.. But this way is not safe, because it can
by-passed by CTRL-ALT-DEL while the first booting
screen (just mouse seen, the empty screen)...
So, I tried to change Windows' SHELL property..
My program runs as well, but then, I can't start
windows' standart EXPLORER (Start Menu and system
tray section)... When I execute Explorer.EXE from my
application, it runs as the Windows Explorer window...
How can I begin START MENU if I change the shell?
Thanks...