Excel 2003 Auto Sort

Maybe:

Private Sub Worksheet_Change(ByVal Target As Range)
If Not Intersect(Target, Range("M2:M" & ActiveSheet.UsedRange.Rows.count + 1)) Is Nothing Then
        Range("M2:M" & ActiveSheet.UsedRange.Rows.count + 1).Select
        Selection.Sort Key1:=Range("M2"), Order1:=xlDescending, Header:=xlGuess, _
        OrderCustom:=1, MatchCase:=False, Orientation:=xlTopToBottom, _
        DataOption1:=xlSortNormal
End If
End Sub

For 2003 Tools>Macro>VBA Editor-select the sheet on the right side> on the left side drop down Choose Worksheet, Right side drop down Choose Change. Paste the code there.

BTW, Welcome to the Forum.