Prerequisites for Outlook 2007 (see 2010, 2013 below)
To install this quickly,
choose 4) RSA and key size of 2048 default will do, or 4096 for better security, and continue, it's pretty logical.
OutlookGnuPGGnuPG addin for Outlook 2007
Here is the Github project :
NOTE: To get this to work in Vista/7 you have to run the installer as administrator. Find your command shell in the run menu, right click and select run as administrator. Then do msiexec -i "path\and\filename.msi". That should get you past the impersonation difficulties.
PGP for Outlook 2010, 2013
A simple OpenPGP encryption plugin for Outlook 2010 and 2013.