If you already have the outline/subtotals/autofilter applied, you can protect
the worksheet in code (auto_open/workbook_open??).

Option Explicit
Sub auto_open()
   With Worksheets("sheet1")
       .Protect Password:="hi", userinterfaceonly:=True
       .EnableOutlining = True
       '.EnableAutoFilter = True
   End With
End Sub

It needs to be reset each time you open the workbook.  (Earlier versions of
excel don't remember it after closing the workbook.  IIRC, xl2002+ will remember
the allow autofilter setting under tools|Protection|protect sheet.)

If you're new to macros, you may want to read David McRitchie's intro at:
http://www.mvps.org/dmcritchie/excel/getstarted.htm