That's a SQL 2000 instance, not SQL 2005 (run Select @@version to prove it).
AdventureWorks is a 2005 or 2008 database and can only be attached to those versions. Downgrade of a database is never possible.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability