How do I generate a sequential serial number every time I tab to a new line?

I am trying to create a chart which would auto serial number every time I start a new line.

Example,


000-001
000-002
000-003
etc etc.

I only want the serial number to appear when a new line is started, not before.


I tried a VBA code, which worked for a while but for some reason it has stopped.

Private Sub Workbook_Open()

Sheets(1).Range("A1").Value = Sheets(1).Range("A1").Value + 1
ActiveWorkbook.Save



End Sub

I could really use some help!

I am using excel 2007 at work. I know that Access would probably be better but unfortunately I am using a work computer and the have Excel.