If the popup was caused by an avast! daemon crash (which is the most probable reason), then you will also have a crash report under “DIAGNOSTIC AND USAGE INFORMATION”.
I have a similar problem. It says that it could not connect to the antivirus engine and won’t run a scan. The only thing that still works is the online security part of the program. I tried uninstalling it, restarting my mac, everything. I even downloaded a different antivirus software and it had the same problem.
Is there anything that I need to do to get it up and running again?