Create a table in Access, holding the login info. (You might want to build your own encryption routine to make these values hard to decypher.)
When a user logs in (that is, fills in a username and password textbox), try to open a recordset on that table
Code:
...
"WHERE USERNAME = """ & UName.Text & """ " & _
" AND PASSWORD = """ & PWord.Text & """"
If the recordset is empty, they don't get logged in.