Try upgrading to SQL Server 2008 R2 SP2, you can find the patches here:
http://support.microsoft.com/kb/2527041
Was your backup created in SQL Server 2005 and you are trying to restore it to SQL Server 2008 R2 (without SP1/SP2)?
In some cases I have noticed that if you take a backup from SQL Server 2005 and try to restore it into SQL Server 2008 it will fail if you don't have SP1/SP2 installed (though I'm not sure why)...