Checking or unchecking the "Do not use Chromes as avast! default browser" doesn't seem to do anything!
While testing this I uninstalled Google Chrome and opened Avast! and performed update checks and went to the Support Tab and clicked on a few buttons that would open up links previously in Chrome by default, but now they had switched over to the system default browser and used that, so I checked/unticked the "Do not use Chromes as avast! default browser" a few times and after re-boots for nothing while Chrome was no longer present on my system.
Once I returned Google Chrome back onto my system both checking & unchecking the "Do not use Chromes as avast! default browser" didn't do anything to return Avast! to use Chrome by default again, even after re-boots and I even restored Avast! back to original default settings in the troubleshooting tab. Bah!
It seems that only a full complete re-install of Avast! a fresh with the option to both install and use Google Chrome by default for Avast! accepted will work, that check box does nadda unless I'm missing a trick.