GNU Privacy Guard (GnuPG) is available for OS X and Windows.  It can
do both public key and symmetric cryptography.  Sounds like you only
need symmetric if you have a secure channel for passing the key.

https://www.gnupg.org/download/
https://gpgtools.org/

Indefinite trial.

Mail, the email client on OS X, can also do encrypted mail.  You may
need to find a client for Windows which will do it.

Other method I use is to upload the file to my web server in a
directory that cannot be discovered, then pass the URL only by telephone
or face to face.  After they download, be sure to remove the file, so
that third parties (service provider caches, virus content evaluators)
don't get it by accident when they review the logs kept by the person.

--=20
James Cameron
http://quozl.netrek.org/
--=20
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.