At work we use PGP / GPG encryption when sending around anything that could count as a little sensitive - such as quotes, estimates, usernames and passwords.
Unfortunately, as I'm running on Vista this has proved difficult to get any of the existing suites to behave. The best was the PGP Desktop suite, but it literally takes over - and slows the machine down horribly.
The OpenPGP package kind of works. But the Outlook plugin doesn't correctly sign / encrypt the email - well, it does, but not "nicely".
So unfortunately I'm left without PGP / GPG support on my windows machines again - and am back using it through KMail and Evolution on my development Linux box. Down right annoying.
I did however fine Sharp Privacy ... Now all I need is time!