Someone asked me the other day if it was possible to add a password to the Notes section of their Outlook software. Unfortunately this isn't possible, but what you can do is add a password on the whole data file which the program will prompt you for when you try to open it up.
Without the correct password, people cannot view your e-mail, contacts, calendar, tasks or notes.

How to set a password on your Outlook data:

To set-up a password, open up Outlook. Once loaded, single left click on 'Personal Folders' in the right hand 'Folder' window. 'Personal Folders' should now be highlighted. Right click on 'Personal Folders' and choose the 'Properties for Personal Folders' menu option. 
In the new window that appears, make sure you're on the 'General' tab. Click the 'Advanced' button. Press the 'Change Password' button. Don't type anything in the top box (current password) but type your chosen password into the 'New Password' and 'Verify Password' boxes. Make sure that the 'Save this password' box is NOT ticked (otherwise you won't be prompted to enter it, defeating the object). Press 'OK' and close off all the windows.
When you restart Outlook you should be prompted for the password.

How to remove the password:

To remove the password, follow the steps above until you get to the point of entering the passwords. Input your current password in the top box, then leave the 'New Password' and 'Verify Password' boxes blank. Press 'OK' and close all the windows. When you restart Outlook you shouldn't be prompted to enter a password.