Your security "environment" is not attached to any specific MDB file nor does
any particular MDB file determine it.

Whenever you launch Access you are *always* doing so with a workgroup file (MDW
extension) and that file sets the security environment.  If you do not specify a
workgroup file as a command line argument when launching Access then your
default workgroup file is used.

Now; if the workgroup file being used has a password applied to the default user
account "Admin", then you are prompted for a UserName and Password when opening
files.  At this point Access doesn't care what file you are attempting to open.
All it knows is that you are using a workgroup that requires logging in.

What the security wizard did was create a new workgroup file (that requires a
login) AND it made that workgroup your default which is why you are always
prompted.  At some point in the wizard you were asked if this is what you wanted
and you must have responded that this is what you wanted.  Otherwise it would
have left your default workgroup alone and instead it would have created a
special shortcut that you would use when opening your secure file.

So to fix it...

Use the workgroup administrator utility to change your default workgroup back to
System.MDW.  This should allow you to open all non-secure files without being
prompted.  Then you can create a shortcut for your secure file that uses a
command line argument to specify a non-default workgroup.  The syntax for such a
shortcut is generically...

"Path to MSAccess.exe" /wrkgrp "path to secure MDW file" "path to secure MDB
file"

You still have another problem though because even though you are currently
being prompted every time you try to open a file, the UserName that you created
during the security wizard should get you in.  Did you write this information
down?  Have you tried using that UserName with a blank password?