How to Encrypt a Spreadsheet in Excel 2003

1. Create a strong document password. After opening the file you want to encrypt in Excel, click the 'Tools' item in the menu bar at the top of the screen, then select 'Options.' In the dialog box, click the 'Security' tab. Enter a strong password in the field labeled 'Password to open.' Strong passwords are at least 10 characters in length, not easy to guess, and include a combination of letters, numbers and symbols.
2. Set the document encryption type. Click the 'Advanced' button next to the password field you just completed. Excel presents you a list of cryptographic algorithm collections used to secure spreadsheets. Encryption of 128 bits or more is considered strong, but some encryption types only support 40 or 56 bits. Scroll down the list and click the 'Microsoft Strong Cryptographic Provider' option, then enter a value of 128 in the field labeled 'Choose a key length.' Click 'OK' to close the dialog box. Click 'OK' a second time to close the Options box.
3. Save your document. Click the 'File' menu at the top of your screen, then select 'Save.' Your Excel spreadsheet is now protected with a strong password and 128-bit encryption.