The optimise for ad-hoc workloads option does not exist in SQL 2005. It was added in SQL 2008.
p.s. you may want to change 'allow updates' back to 0 if you want to get rid if that 'adhoc updates to the system catalogs are not allowed' message every time you try to reconfigure.
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