Nvm. It works.
Here is the final code:
Private Sub Knop19_Click()
Dim sMsg As String, sOldWW As String, strSQL As String
Dim iResponse As Integer
If Len(Nz(Me.txtOld, "")) = 0 Then
MsgBox "Voer het huidige wachtwoord in.", , "Huidig wachtwoord"
Me.txtOld.SetFocus...