Protect them with LockXLS

You can apply different protection options to your Excel documents:
Hardware based Activation Code (the workbook is locked to a specific PC). More ...
As an alternative to locking the workbook to a specific PC, an Activation Code may be bound to a USB flash drive. This flexible way allows your customer to use activated software on the any PC he need.
Serial Number, Trial period, Password
Protect without activation
You can create rental files using LockXLS. The hardware based Activation Code will unlock the spreadsheet only for a specified period of time.
You may limit an evaluation by the number of openings of a protected document.
Workbook can be compiled to executable file, which doesn't have external dependencies. It doesn't require any additional libraries to be installed on the customer's PC.
Formulas and VBA code are unavailable to user. Customer will not be able to view and analyze the logic of your workbook.
Formulas and VBA code are inaccessible through COM Object Model. User cannot write a VBA code to retrieve your formulas or macros.
Locked workbook has End User License Agreement, Splash Image, Registration Reminder, Custom Help file, About Box.
Customize the activation messages shown when the user opens a locked workbook.
Data on hidden sheets cannot be accessed from an external application.
There are no limitations to the workbooks, which can be protected. All features of Microsoft Excel are supported by LockXLS.

Home:
http://www.lockxls.com/product.asp