How to make SQL Server 2008 compatible to SQL Server 2000
According to the following link there is a step missing: 

http://blogs.technet.com/b/mdegre/archive/2012/06/15/migration-sql-server-2000-to-sql-server-2012.aspx


You must first update your SQL Server 2000 by upgrading to SP4 before making a backup to restore onto SQL Server 2008.  


The TechNet link refers to moving from SQL Server 2000 to SQL Server 2012, but the first step is moving from SQL Server 2000 SP4 to SQL Server 2008.

If going all the way to a current version of SQL Server you will have to make the migration in two steps:


Step 1: Make a first migration from SQL Server 2000 to SQL Server 2008 for instance. You need to be on SQL Server 2000 SP4.
Step 2: Make a second migration from SQL Server 2008 to 2012.