You can use free open-source software like
GnuPG - the installer comes with a plugin for Outlook and also a complete email client (Claws) which supports encryption. You can then configure either Outlook or the provided Claws email client to use your Yahoo or Gmail account. Your email can be encrypted so it's unreadable by anyone who doesn't have either your public key or a supercomputer to break the encryption.
There's also a Firefox plugin which lets you encrypt Gmail messages.
Personally, I don't bother, since most email recipients will not go to the trouble of setting up email clients which can read encrypted email - it's best to remember that email is insecure and can be read by anyone with access to the SMTP servers routing your email to it's destination, including Gmail and Yahoo SMTP servers.