Transferring databases from SQL Server 2008 to SQL Server 2005 is not possible with standard backup and restore facilities.
Leverage the scripting wizard in SQL Server 2008 to script data as well as schemas in SQL Server 2005 compatibility mode.
Using the „Generate SQL Server Scripts“ Wizard
The Northwind database is no longer shipped as part of the SQL Server installation, but you can download it from go.microsoft.com. The data is scripted as INSERT statements.
To create the scripts, you have to run the „Generate SQL Server Scripts“ wizard, which you can run within SQL Server Management Studio (once Object Explorer is connected to the appropriate instance) by right clicking on the database and selecting „Tasks –> Generate Scripts.“