Password Protect Microsoft Office 2003 Documents

1. Open the Document you wish to encrypt / password protect

2. Click Tools, then Click Options

3. Click Security tab, and then Click Advanced button

4. Scroll down and Click RC4, Microsoft Enhanced RSA and AES Cryptographic Provider. By default the Key Length is 128 and the box Encrypt document properties is checked. Verify this and Click OK

Source: http://www.groovypost.com/howto/microsoft/office/password-protect-microsoft-office-2003-documents/