Here is a technique that should help you find the problem.
Determine what code executes for startup. It will either be a function
identified in a macro called Autoexec or in a form called by startup. You
can check Tools->Startup from the menu, and see what is in Display Form/Page:
Once you have that determined, open the code module for the form, or the
module the function from Autoexec is in and set a breakpoint on the first
line of executable code. Then open the startup form or call the function
from the immediate window. Then when it tries to open the mysterious form,
you will see what the name of it is and where it is called.
If you know the name of the form, then you can go into Tools->Options and
check Show hidden objects on the view tab. It may be that the form is hidden.