Restore a SQL Server 2008 Database into SQL Server 2005
Backing up databases in SQL Server 2008 and then restoring them into SQL Server 2005 is not directly possible. You'll learn quickly that the database backups are not backward compatible.
However, with SQL Server 2008 Management Studio, you can script data and schemas in SQL Server 2005 mode and port your data over using the following steps....
Steps Creating Scripts to Move SQL Server 2008 to SQL Server 2005
Steps to Generate Scripts to Move SQL Server 2008 to SQL Server 2005