Hi 
I’ve seen this too! 
It’s happened to me approximately 3 times in the last week.
But I also have another Avast problem of websites not loading because of the Avast setting - Enable HTTPS Scanning.
This is a problem that’s apparently only affecting XP and Vista.
Note… I have absolutely no idea if the two problems are linked or separate.
It’s just strange that I have no problems with Avast for years and years. Then two problems in just one week.
The shields down on boot up problem has not happened for the last two days since turning off Enable HTTPS Scanning. But that could possibly be a complete coincidence. As the shields down on boot problem does seem to be quite random.