Think of yourself as the *only* program that the user can see.  You show the
only user interface visible on the device.  It's just your window, no start
menu, no desktop, no task bar, no system tray.  Normally, you set your
window to run front-most and you disable and hide the task bar, preventing
Ctrl+Alt+Del and Alt+Tab sorts of things from doing anything and, since
you're in front, even the buttons on the outside of the device, though they
may launch programs, will not allow those programs to be seen, because your
window will be in front of them.

One of your program's features, then, will be some means of entering
Administrator Mode.  When the user does that (you decide how that happens,
whether with a password, or a fingerprint scan or whatever), you change your
window's settings so that it's no longer forced to the front, and you
reenable and show the task bar.  Now, the guy can do anything that he could
do normally on a Pocket PC device.  When he's done with administrator mode,
he switches back to user mode and you hide and disable the task bar, again,
and make your window front-most, again.

Paul T.