When upgrading from SQL 2000 to SQL 2005 you must follow the edition upgrade rule. The rule says that you must upgrade to the same edition or higher.
When upgrading SQL 2000 Standard, you must upgrade to SQL 2005 Standard or SQL 2005 Enterprise. This creates a problem with the SQL 2000 which is included with SBS. The SQL 2000 which came with SBS is SQL 2000 Standard, but the SQL 2005 which comes with SBS is the Workgroup edition which is a lower edition.
Are you sure you have Standard Edition installed? What edition are you trying to upgrade to?