how can use Tranfer SQL Server objects task to convert Mssql 2008 database to 2005 ?

1. Generate Scripts for each database, and you have an option "Script for Server Version" and Select SQL Server 2005. To generate script , Right DataBase > Generate Script > continue on screen instructions. Execute the scrips on destination server.
 
2. Use SSIS to transfer all data
 
Also see : http://www.sqlservermart.com/HowTo/MSSQL_Convert_2008_To_2005.aspx[^]