You need an 64 bit CPU (and OS X) to run Avast. Your Mac is apparently an old 32bit one, so it can not run Avast.
Unfortunately, there is no usable Avast version that would run on such system, sorry.
You may try to find some other AV that can run on such system, but AFAIK Avast is the AV with the best OS support
amongst the Mac AVs (most others require OS X 10.8 as the minimum), so you will most probable find anything...