Cluster Security Policy
Use the Cluster Security Policy page to configure the security policy for your failover cluster instance.
Specify global or local domain groups for clustered services. All resource permissions are controlled by domain-level groups that include SQL Server service accounts as group members. For information about service security IDs (SID) functionality on Windows Server 2008, see Setting Up Windows Service Accounts.
If you are installing a SQL Server 2008 Failover Cluster Instance in a Windows 2000 mixed mode domain, you must use domain global groups for SQL Server Clustered Services.
Windows 2000 domain controllers can operate in mixed mode and native mode. Mixed mode allows downlevel domain controllers in the same domain.