My primary concern is that Avast will detect at some point before infection, such as upon opening the attachment. I was afraid to go that far to test it.

I use Outlook Express. Avast is scanning my email and is set to high. It even put the “message is clean” at the bottom. I saved the attachment and scanned by right-clicking on it and selecting scan. It showed that it scanned 3 files in the zip file and still did not detect anything. I then scanned the zip with Mcafee FreeScan and the infection was detected.