How to secure your emails with GnuPG and Enigmail
With the GNU Privacy Guard (GnuPG) and a good mail client software like Mozilla (web+email) or Mozilla Thunderbird (just email), you can have really secure email conversations, using excellent and free software. Plain emails have indeed two big problems: they are neither authenticated (you can pretend you are anybody when you send a mail, which is used by spammers and viruses), nor encrypted (anybody on the path your email takes, from you to your correspondent, can read it). With GnuPG-powered email signature and encryption, you can be sure the messages are from the person they claim to be, and be sure only the ones you want will have access to your emails!
Read the Enigmail how-to
Be sure to use the latest version of Enigmail. Version 0.83.5 version works with Mozilla 1.6, 1.7a, and Thunderbird 0.5. In addition, new binaries are available for the latest nightly builds of Mozilla 1.7b and Thunderbird 0.6a.