Backing up databases in SQL Server 2008 and then restoring them into SQL Server 2005 is not directly possible. There is a process to generate script….
1. Use the scripting wizard in SQL Server 2008 to script data as well as schemas into SQL Server 2005 compatibility mode
2. Run Generate SQL Server Scripts wizard in SQL Server Management Studio (Object Explorer) by right clicking on database and click Tasks > Generate Scripts
3. Click Script all objects in selected database and then
4. click Next
5. Change script options
Imp NOTE: <b>Set Script for Server Version to SQL Server 2005 and set Script Data to True. </b>If u are putting the database on a new instance for the first time, make sure the Script Database Create option is set to True.
6. Click Next
7. Now Finish
Remember: After creating the file u can run the script in SQL Server 2005 Management Studio to recreate the database.
Hope it may help u !