So Thunderbird is like Firefox except it’s for email. It is a very cool program. It runs on Mac, Windows and *nix. It also handles encryption using GPG. I’m told this works on Windows just fine, too, once adequately configured. I’m dying for an easy way to get Windows people using GPG encryption and Thunderbird might be a good option. A couple of good links I found:

How to secure your emails with GnuPG and Enigmail
A Practical Introduction to GNU Privacy Guard in Windows