You need to either use ULS or create your own login screen for users. But I would split the database first in any case. Make a copy of it before you do so, so that you can rollback if needed.
I just set this up on a 2007 database. I set up an input form with unbound controls for user name and password. I also set up a hidden table with passwords listed. When the enter button on the input form is pressed it runs code that does a dlookup on the password table of the password entered based on the user name entered. If passwords match, it opens the main menu. If there's no match it pops up a message saying the entries are invalid. You also need to set this input form up as your startup form for this database. Let me know if this is what you were looking for and I'll post the code