Dim OldValue

Private Sub Combo1_Click()
With Combo1
    Text1 = LTrim(Str(Val(Text1) * 10 ^ (OldValue - .ItemData(.ListIndex))))
    OldValue = .ItemData(.ListIndex)
End With
End Sub

Private Sub Form_Load()
With Combo1
    .AddItem "мм"
    .ItemData(.NewIndex) = -3
    .AddItem "см"
    .ItemData(.NewIndex) = -2
    .AddItem "дм"
    .ItemData(.NewIndex) = -1
    .AddItem "м."
    .ItemData(.NewIndex) = 0
    .AddItem "км"
    .ItemData(.NewIndex) = 3
    .ListIndex = 3
    OldValue = .ItemData(.ListIndex)
End With
End Sub