For SQL Server 2005,I also think you need to exclude the predefined database users,dbo and sys.The query needs to be modified as follows:

select u.name from master..syslogins l right join sysusers u on l.sid = u.sid where l.sid is null and issqlrole 1 and isapprole 1 and (u.name 'INFORMATION_SCHEMA' and u.name 'guest'

and u.name 'system_function_schema' and u.name 'dbo' and u.name 'sys')