I assume your using Excel 2007?
If yes, just follow the steps mrgroove posted here:
Configure Password to Open or Password to Modify on Excel SpreadsheetAlthough the demo shows word screenshots, the same is true for Excel. Basically what you want to do is set a MODIFY password.
This will allow users to open your excel spreadsheet with no password and make changes however they will need to save the changes to a NEW document unless they know the MODIFY password.
This is a good trick for creating a template. That's what I do at work.