I recommend you take a look at PGPfone. It is available in Mac and Windows versions and is free. As the name implies, it also has the merit of keeping your internet telephone calls private. While it might not defeat the latest government computers, it should certainly keep your conversations private from lesser or more casual snoops. If you live in the US or Canada you may get a copy from the MIT site:
http://web.mit.edu/network/pgp
If you live elsewhere you may obtain it from the PGP International site:
http://www.pgpi.org/
Good luck!