Here is a KB article describing what the BUILTIN\Administrators login is:

http://support.microsoft.com/kb/263712

and here is a blog post about removing the group (which is a bigger deal on 2000 and 2005):

http://www.mssqltips.com/sqlservertip/1017/security-issues-with-the-sql-server-builtin-administrators-group/

To review who is in 'BUILTIN\Administrators' use the following:

EXEC xp_logininfo 'BUILTIN\Administrators', 'members'