Here are instructions for encrypting Microsoft Office documents:

Office 2007:

1. In Word or Excel, click on the Microsoft Office Button and select "Prepare->Encrypt Document".
2. Enter a strong password, click "OK" and re-enter it for verification.
3. When you save the document, it will be encrypted with the password. The encryption uses AES 128-bit strong encryption by default.

Office 2003:

1. In Word or Excel, click on the "Tools" menu and select "Options".
2. In the window that pops up, select the "Security" tab.
3. Click on the "Advanced" button to open a list of encryption options.
4. Select "RC4, Microsoft Strong Cryptographic Provider", make sure that "128" is entered beside "Choose a key length", and make sure "Encrypt Document Properties" is selected.
5. Click OK to return to the security options box.
6. Enter a strong password in the "Password to open" field.
7. Click on "OK" to close the security options box.
8. Re-enter the password to verify it.
9. When you save the document, it will be encrypted with the password.