Currently, the official Avast for Mac Edition available for download is still at version 2.74, released somewhere in 2007. This version still has many annoyances and bugs that have not yet been resolved (one of these issues is the ability to run automatically in the background, without the main window popping up at start up each time).
Luckily, the Avast Forum has a post, providing a new release candidate is available for download for some time now. The build is also a bit outdated but has at least many fixes that aren't present in the official release.
The direct download link is: http://public.avast.com/~cimbal/avast.3.11.zip.
