Something's got wrong, probably.

Can you, please, send me files:
avast4\setup\setup.ini
avast4\setup\setup.log
and directory listing of avast4\setup. You can make it from command prompt by
dir *.* >list.txt
Please also tell me how long is the iso file.
Doesn't re-creating the iso help? (I suggest using CD-RW for such tests, if you don't want to get a pile of toasters).
my email is kubecj AT asw DOT cz
thanks,