If the messages are stored on aol's servers and not on your PC, the 2 things to do are:
1. Use a good password for your aol-account and tell it to nobody.
2. Hope they have a good protection against hackers with aol.
The best protection of Microsoft works documents is to copy them to a USB-stick, delete them from your hard disk using any 'erase' program and store that USB stick in a very safe place (along with your diamonds and 1000$ bills).
If you want to keep them on your PC, there's an option to make them private so that not even anybody else logging in as administrator can see them. But be sure the prying eyes of the FBI and the NSA won't have any trouble to read it. But combined with a good password on your Windows account and on the secret administrator account it suffices for day-to-day use. We never recommend encryption software, because we see too much help requests here about 'help, I forgot my encryption password' or 'I can't read the file on my new PC'.