Jsc3489,

I think you probably want this...

 If DLookup("[Password]", "UserDef", "User_Name ='" & Me.Login & "'") =
Me.Password Then

So, your UserDef table has 2 fields, User_Name and Password.  And the
form from which this code is being called has an unbound combobox called
Login, for the selection of the user name, and an unbound textbox called
Password for the entry of the user's password.  And I would guess the
rest of the code might look something like this...

 If Me.Password = DLookup("[Password]", "UserDef", "User_Name ='" &
Me.Login & "'") Then
     DoCmd.OpenForm "YouCanDoSomethingNow"
 Else
     MsgBox "Invalid password"
     Me.Password = Null
 End If

At the risk of adding confusion, an alternative approach would be to
include both columns of the UserDef table in the Row Source of the Login
combobox.  In this case, the Properties of the combobox get set like this...
 Column Count: 2
 Bound Column: 1
 Column Widths: x;0
    (where x is the number that is wide enough to show the user names
in the drop-down list)
... and then you can dispense with the DLookup function, in favour of...

 If Me.Password = Me.Login.Column(1)