How to: Rebuild the Registry for SQL Server 2005
To rebuild the registry, from the command prompt run the Setup command with the REINSTALL=ALL,REINSTALDATABASE=1, and REINSTALLMODE=omus properties. This will rebuild, verify, and repair a Microsoft SQL Server instance. This procedure is most often used to rebuild the registry for a damaged installation of SQL Server.
To rebuild the registry, you must use the same package file and options that you specified during the initial installation. If you do not know this information, uninstall and then reinstall SQL Server, rather than rebuilding the registry.
When possible, use Windows Authentication. Do not leave the sa password blank; always use strong passwords.
For more information and sample command prompt syntax, see How to: Install SQL Server 2005 from the Command Prompt.