gpg4win[0] works -- or it did the last time I used it anyway (several years ago) -- and from a quick search I found "Outlook Privacy Plugin"[1] (for 2010 and 2013).

Unfortunately, the best solution I've found for Windows is the commercial PGP product[2]. It's not free or open source but it does work and it configures itself for opportunistic encryption, so that's a plus.

[0]: https://www.gpg4win.org/ [1]: https://code.google.com/p/outlook-privacy-plugin/ [2]: https://www.symantec.com/desktop-email-encryption