In SQL Server 2000 you should be using the INFORMATION_SCHEMA views. In SQL Server 2005 and up you should refer to the system views. Starting in SQL Server 2005 there are no more system tables. Everything is held within these system catalog views. They are the views which are in the sys schema.
Eventually the INFORMATION_SCHEMA views will be removed from SQL Server, so it’s best to begin using the system catalog views now.