I had the same problem until i downloaded this: http://www.microsoft.com/download/en/details.aspx?displaylang=en&id=35
I used it to solve a DX error for The Witcher, but apparently it solved Assassin's Creed, too.
That link is for the web-based installer and it frequently fails due to an error with one of the cab files. If you encounter that, you can download the whole thing locally at http://www.microsoft.com/download/en/confirmation.aspx?id=9033
I haven't kept up, but I think June 2010 is the latest DirectX End-User Runtime available: http://www.microsoft.com/download/en/details.aspx?displaylang=en&id=8109
From what I understand it includes earlier versions, which should also apply to the Feb 2010 version you linked to.
For some reason I was under the impression that there was an even newer version available, but not according to Microsoft's site: Web installer version
Version: 9.29.1974 Date published: 4/18/2011 DirectX End-User Runtimes (June 2010)
Version: 9.29.1974 Date published: 4/18/2011