Hi Cyber,
First of all, I think you should know how encrypted mail is implemented.
A sender encrypts his mail with his private key. And if the recipient want to open the encrypted mail, he need to get the sender's public key.
The recipient can get the sender's public key through import the sender's certificate manully . Or the sender sends a mail with his certificate and so on.
So I think you should ask a question in other Forum like windows XP for details.
http://social.technet.microsoft.com/Forums/en-US/itproxpsp/threads
Frank Wang