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This behavior is by design. In addition to adding the user account that is running the setup as a local administrator as documented in How to: Install SQL Server 2008 (Setup), the setup user account requires the following default user rights for the setup to complete successfully.
Local Policy Object Display Name (User Right)
- Backup files and directories (SeBackupPrivilege)
- Debug Programs (SeDebugPrivilege)
- Manage auditing and security log (SeSecurityPrivilege)