first of all it would help, if you said what you want to do.

>  I got a certificate and installed it in Outlook. Then I signed a
message
> and sent it to my Unix box.

there remains the question what kind of certificate you mean. different
kinds and different uses for each kind.

> I received the e-mail just fine. How do I
> extract the public key to store on my end?  Are the users certificate
and
> users public key different things?

well, usually the certificate is the users public key encrypted with the
certification authorities private key. therefore you can easily gain the
users public key by decrypting the certificate with the known public key
of the CA. this procedure of encrypting all (or a hash value) of a
message with a private key is also known as signing the message or
certificate.
also since only the CA itself should have access to their private key,
you can be shure that the public key you received from the user really
belongs to the user if (and only if) you trust the CA's procedures to
validate the users id.

as for your actual problem it would really help if you stated what you
wanted to do and what you did (where did you get the cert from and what
kind of cert).

joachim

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