incrementing account numbers

To enable macros:

Tools>Macro>Security

In Security Level Tab, set to Medium.

For the second cell. Just add the other cell in the macro:

Sub Auto_Open()
Sheets("whicheversheet").select
Cells(1, 5).Value = Cells(1, 5).Value + 1
Cells(19, 5).Value = Cells(19, 5).Value + 1
End Sub