replacing the current machine

I have already a machine running as win2003 DC and Exchange 2003, lately it start to hang or restart and giving error messages(2102, 8026, 8026,?)
I’m thinking of adding anew machine to implement exchange high availability to limit the down time when the first machine and use 3rd party application to make replication between the two machines

I’m new to this .and need suggestions about the right way to do this
(if I need to migrate mailboxes or install 2nd DC if needed to support authentication if the first machine is down or any other ideas)
Plz help