Your selection criteria in your DLookups are incomplete. You should probably use something like this (Assuming that the value in [Combo0] is the user name):
If Me.PASSWORD.Value = DLookup("USERNAME", "SYS_USER", "[username]='" & me.[Combo0] & "' and [password]='" & Me.PASSWORD.Value & "'") That way you restrict the row checked against the combination of selected user and supplied password.
Please note that this method will have to be used everywhere you are using "[password]=" as your selection criterion.
Also, from the MSAccess VBA Help File:Tips
Although you can use the DLookup function to display a value from a field in a foreign table, it may be more efficient to create a query that contains the fields that you need from both tables and then to base your form or report on that query.
Hope this helps.