Here is what you do in Excel 2003:
Here is what you do in Excel 2007:
Once this is done, the sheet is hidden and cannot be unhidden without the password. Make sense?
If you really need to keep some calculations secret, try this: use Access (or another Excel workbook or some other DB of your choice) to calculate what you need calculated, and export only the "unclassified" results to your Excel workbook.