Repair steps for Exchange 2007 on SBS 2008 have been posted to the SBS TechCenter. These steps cover reinstalling the Client Access, Hub Transport, and Mailbox Server roles for Exchange 2007. You can view the documents here: http://technet.microsoft.com/en-us/library/dd728006.aspx. Please note that removing and reinstalling Exchange roles on SBS 2008 should not be a common troubleshooting step. These documents should only be used when other troubleshooting resources have been exhausted. Make sure to take a full backup of the server before performing any of the steps.