You can't restore a newer/higher version of SQL Server to an older version (e.g. 2008 R2 --> 2000). You can go the other way (older to newer), though that doesn't help you in this situation.
That being said, how large is the database? Can you script out the database and tweak the SQL to create the DB structure on the 2000 server (being careful to remove any newer features not in 2000). Then perhaps use SSIS to move the data from the 2008 R2 server to your newly created 2000 db.
Just a thought,