For users on a PC, your choices are many but below are the most popular choices:

You can download a 30 day demo of PGP here: (PC & MAC Compatible with support)
http://www.pgp.com/downloads/desktoptrial/desktoptrial2.html

After 30 days you will need to purchase the software.  It runs 99.00 for pgp desktop home.

A compatible free GPG/GnuPG program is also acceptable:
http://www.gpg4win.org/  (PC Only, personally tested with Vista/XP)

This software works best with Mozilla ThunderBird:
http://www.mozillamessaging.com/

With an addon called “EnigMail”:
http://enigmail.mozdev.org/home/index.php

James