Ask Slashdot: Cryptography in Mail software?

Bartmoss asksL: "Obviously, nobody will use encryption if two problems occure: (a) your friends won't be able to read your mail because they don't have crypto, and (b) your software doesn't have crypto. I'm wondering - are there good HOWTO's and info sites on how to plug encryption into leading mail software for UNIX, Mac and Windows? What Windows-Software supports PGP, and which can have PGP support added? Does anybody have information on clients people could use for crypted mails?"