Hi, welcome to the forums.
Assuming that you installed avast! in the default folder, this is the path needed to scan downloads, C:\Program Files\Alwil Software\Avast4\ashQuick.exe
This of course doesn't take into consideration if PKzip requires a switch after the path (e.g. %f or %d or %1, etc), for that you will need to check the PKzip instructions/help.