While reading this I realized I had seen this question before. I got this from Aaron T. Blood at www.XL-Logic.com a while back. Seems to work. It uses a textfile and it increments each time the file is opened, whether you save it or not.

Sub Auto_open()
On Error GoTo ErrorHandler
One:
Open "c:\Counter.txt" For Input As #1
Input #1, x
Close #1
x = x + 1
Two:
Sheets(1).Range("A1").Value = x
Open "c:\Counter.txt" For Output As #1
Write #1, x
Close #1
Exit Sub
ErrorHandler:
    Select Case Err.Number
    Case 53  'If Counter file does not exist...
        x = InputBox("Enter Number to Begin Counting With", "Create 'Counter.txt' File")
        Resume Two
    Case Else
        Resume Next
    End Select
End Sub