you've got the Adventureworks DB for SQL Server 2008 R2 which can't be attached to SQL 2008.
see this thread with the same issue http://social.msdn.microsoft.com/Forums/en-US/sqlserversamples/thread/52303c10-1bf5-4522-a35a-ad6cd283c2ff
in this thread the answers contains the correct link and instruction to install Adventureworks on SQL 2008
http://social.msdn.microsoft.com/Forums/en-US/sqlsetupandupgrade/thread/214d291e-0aa6-4600-ac38-72e7870bdf3c
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