When Microsoft Search Server Express 2010 is installed and a server farm configuration is selected, a domain account is required. When the server is not on a domain, some administrators may entertain the idea of adding the domain controller role on the search server.
When Search Server is configured as a server farm, a domain account is required. The domain controller (DC) role should not be added to a server where Microsoft Search Server will be installed. The DC role prevents the creation of local security groups and impacts updates to Search Server.
The best approach is to provision a separate server as a domain controller.