Private Sub UserForm_Initialize()
Dim aRow, i As Long
Application.EnableEvents = False
ComboBox1.Clear
aRow = [A65536].End(xlUp).Row
ComboBox1.AddItem "Wähle die Produktnummer und Seriennummer"
For i = 2 To aRow
    ComboBox1.AddItem Cells(i, 1) & ", " & Cells(i, 2)
Next i
ComboBox1.ListIndex = 0
Application.EnableEvents = True
End Sub

Private Sub ComboBox1_Click()
If ComboBox1.ListIndex <> 0 Then
    TextBox1 = Cells(ComboBox1.ListIndex + 1, 1)
    TextBox2 = Cells(ComboBox1.ListIndex + 1, 2)
    TextBox3 = Cells(ComboBox1.ListIndex + 1, 3)
Else
    TextBox1 = ""
    TextBox2 = ""
    TextBox3 = ""
End If
End Sub