In order to make sure your important office files are not easily accessed by others, you can add a password to these files using the following process. Iam a user of Microsoft Office 2007, hence this post would be for the users of this version.
Preparing your File for Password Protection
1. Encrypting your file – In order to encrypt your file you need to click on the ‘Microsoft Office Button’ located on the top left of your MS Excel, and select the prepare option to ‘Encrypt Document’.

2. Once you click on this you will be asked to Enter your password for this file twice for confirmation. Make sure you enter a secured password[password strength] upto 255 characters long.

3. Next time when you try to access this file you will be asked to enter the password, only after which you can view the contents, modify the content.

Encrypting Document does hide our valuable data.
Thanks for help?