After reading the following, quoted from MacFixIt, it may explain why my Reader installation was successful the first time around.
QUOTE
We've since discovered a workaround. In the International preference pane of the system preference of Mac OS X, in the language tab, specify a language other than English (or your current default language) at the top, log out, log in using the language you selected. Then change the setting back to your default, log out and back in, and Reader should launch properly.
When I first got my G4 with Jaguar installed I ran across a little application that strips foreign language support from the OS, leaving essentially only on language on the system. That could be why mine worked, but I'm not sure.
At any rate, I think the Reader installer is faulty at best.