This question is somewhat unclear. For example, is “SBC” a typo for “SBS”?
Here is what I can make out of the question so far:
1. You need to rebuild an SBS server
2. You want to backup AD and Exchange and presumably restore it after the server rebuild
1. Perform a full backup of the server, including the system state and Exchange databases
2. Rebuild server
2-1. Make sure to install to the same level of service pack, hotfixes and updates
3. Restore server, Exchange and system state
4. Restart and enter directory restore mode
5. Perform an authoritative restore using NTDS Util
6. Restart and boot into OS
You are likely to encouter a varying degree of challenges along the way that require sorting out but nothing that cannot be overcome.
If the reason that you are rebuilding has to do with OS corruption or anything like that, then backing up and restoring may not be an option in which case you can opt to restore only the system state and Exchange on top of a clean OS install.
*** Minor revision by Wrobinson on 02/02/2008 ***
If you decide to pursue the last option described above, which is to restore AD and Exchange on top of a clean OS install, then you will need to install Exchange Server and replace the default databases with the ones that were backed up.