A zip bomb, also known as a Zip of Death or decompression bomb, is a malicious archive file designed to crash or render useless the program or system reading it. It is often employed to disable antivirus software to create an opening for more traditional viruses.
Rather than hijacking the normal operation of the program, a zip bomb allows the program to work as intended, but the archive is carefully crafted so that unpacking it (e.g. by a virus scanner in order to scan for viruses) requires inordinate amounts of time, disk space or memory.
Avast opens files in a temporary directory to scan them that's why you are seeing them in that folder.
So you have had something nasty on your machine trying to get round Avast could have come from a wesite that Avast uesed its realtime scanner to look at any files attempting to be downloaded.
look at the date of these files
use something like CCleaner to regularly clean temporay files off your machine
I have run Avast for years without problems.