All the assistants are on the Office install CD. If you're in the workplace
it's possible they didn't install them all because you really only need to
have one. They may have streamlined their install to excluse the majority of
them. If this is your home PC, then it's entirely possible you elected to
install only the one so you can simply put your CD back in and add them or
use the Office resource CD.
Additionally, you can download extra assistants from the Microsoft Update
Website. Using the Help menu, click Office on the Web.