Hi, will that be internally for the same domain? In that case you can build a Microsoft CA server (just add the certification authority role on a windows server OS), deploy the CA root to the AD, and then start issuing persoanl certificates to sign and encrypt messages. Once those certs are installed on your clients, you can easily enable outlook2003 to sign or encrypt messages from the compose window, or changing it to default sign or encrypt every new email in the general options.
More info:
http://technet.microsoft.com/en-us/library/cc700804.aspxRgds,
Miguel