To test my macro, set up a table as follows:

5 columns
Columns 2, 3 and 4 contain textinput formfields of the "Number" type with a
default value set to "0". Also, check the "Calculate on Exit" property.
The fields in column 2 also have the following macro (ToggleZero) set On Exit.
Column 5 contains a calculation formfield witht he following formula:
  =SUM(LEFT)
Make sure that each cells contains nothing but one single formfield as
described above.
Do not forget to protect the document before testing it.

Now, here is the macro:

Sub ToggleZero()

With Selection
   If .Cells(1).Range.FormFields(1).Result = 0 Then
       With .Rows(1).Cells(3).Range.FormFields(1)
           .Result = .TextInput.Default
           .Enabled = False
       End With
       With .Rows(1).Cells(4).Range.FormFields(1)
           .Result = .TextInput.Default
           .Enabled = False
       End With
   Else
       With .Rows(1).Cells(3).Range.FormFields(1)
           .Enabled = True
       End With
       With .Rows(1).Cells(4).Range.FormFields(1)
           .Enabled = True
       End With

   End If
End With

End Sub