Excel does implement some 40-bit RC4 encryption method for workbooks that require a password to read (which you can brute force within a day), but the password to modify is trivial to remove.
By default in the old XLS file formats. Office XP introduced CryptoAPI RC4 encryption to improve security, unfortunately the UI for it was removed in Office 2007 and you will have to use the DefaultEncryption registry key to enable it now.