A couple of notes on calculation:
(1) All subsequently opened files will open at the current calculation setting
(2) Calculation settings are stored with the workbook. If you save a workbook as manual, it will open as manual. However, this is secondary to item (1) above.
(3) Since new workbooks in Excel default to automatic calculation (unless you change Book.xls as Andrew suggested), and Excel opens a new workbook upon startup (unless you change the setting), closing Book1 or whatever Excel defaulted to on startup BEFORE opening any files is a good way to ensure the calculation setting as in (2).
HTH