it's wizard driven and if you go in and muck around manually configuring stuff, you will break it and no amount of manually trying to reconfigure it will work.
if you dcpromo demoted, you can not dcpromo promote SBS. you have to do a clean install. Wipe the drive, insert the DVD and run setup. once Windows Server 2003 is installed, the SBS setup will run. It is all wizard driven so just follow the prompts to install SBS. Once you have installed SBS, you must run the CIECW wizard to connect to the internet. The CIECW wiard is located in the server management console, left pane under Things to Do.
If you make any changes to your network, such as switch ISPs, change routers, you must run the CIECW wizard.
Note: if you change from SMTP to POP3 mail, you must run the CIECW wizard. If you change from POP3 to SMTP, you must run the CIECW wizard.
Often, services will report a critical error in the event log, when in fact it's not a critical error, rather, it's the service tried to start before a dependency service has started. often you can ignor these types of errors in the event log if you check under services in admin tools that the service is in fact running. If a service isn't running, check to make sure the dependent service is running. if not start the dependent service, then start the regular service. All this don under services in admin tools.
another note: once you install SBS and complete the wizards, you don't want to go into DNS, DHCP, IIS and start changing stuff. IIS is required for Sharepoint services to work properly and if you uninstall IIS, there's really not way to fix sharepoint so you have to reinstall those components. Even then, it's not a sure fix.
once you install SBS and it runs and does everything it's supposed to do, you don't muck with it. you just leave it alone.