The runtime files can all be installed side by side. They are only files. Windows is programmed to use a certain version set of files, and they are installed at install. Although features are cumulative, you specify as a programmer which ones to import. If I choose Version 8, then I need to install DirectX 8, DirectX 9 will not stand in for it. Different versions of 9, 9.0, 9.0b, 9.0c, are direct replacements, and I import 9, and get the latast, 9.0c. Some new features were introduced, not just security updates, so many programs require 9.0c, as opposed to 9.0b. Windows Vista introduced DirectX 10, Windows 7 introduced DirectX 11, and this was made backwards compatible with Vista in SP2. However, so few graphics cards actually support DirectX 10-11, that it does not really make much difference!
As such, I think the most likely cause of this error is a bad download, and a new (full) download which we can hash if necessary will prove this yay or nay.
Richard
