I know the thread is a little bit old, but hopefully this would help someone someday. I did fall into a bootloop with the Archos 79 Xenon while I was trying to deodex its ROM.
Here's the link to the firmware of the Archos 79 Xenon as provided by their support. I'm downloading it currently, so I can't really tell if it is the original 4.2 or an updated one.
http : // update. archos. com/ AFMv1/ storage/ / files/ full/ a79xe/ 20140708.134233/ update.zip (542MB) [remove spaces, sorry, but the forum rules prevent me from sharing external links, yet