I am getting ready to decomission an old server that has Exchange 2003 on it. At the same time I am getting ready to commission a new server that is setup with Exchange 2003. Both boxes are running Windows server 2003. My problem is how do I migrate the mailboxes, public folders and public calendar from the old box to the new box

Use EMS Move mailbox to move user mailbox from old to new. And use add new server to old server public folder replication list to migrate public & system folder. More information please references KB article Moving mailboxes in Exchange Server 2003 Differences between move mailbox methods How to remove the first Exchange Server 2003 computer from the administrative group

You should also leave the old server in place for a couple of days to allow for all outlook clients to be automatically redirected to the new server. Don't forget to change the mx record nat on your firewall to the ip of the new exchange server before decomissioning the old one and add the new server as a local bridgehead to any connectors you have running.