You have User Access Control enabled. Disable it and try again.
If UAC is on, then being a member of the local Administrators group DOES NOT give you local administrator privileges.
this registry setting does indeed allow all members of the local administrators group to use the admin shares C$, Admin$ etc, on Server 2008.
However if the ACL on a directory is "Administrators: Full control" then members of the local Administrators group (apart from Administrator) still do not have access to the directory even though the ACL grants them access. This is unaffected by the above registry setting.