Thanks lanux128 for pointing me to that other thread. Looking at the AVG commands in the RUN registry key I found that AVG had put ending quotes in the wrong place for one of the components that start at bootup time. This was the apparent cause of the problem.
Still, I am disappointed that AVG failed to respond to my email messages where they should have been able to easily point me to the problem (as I am sure I am not the first person to experience this with their software). I understand that with the free version of AVG, there is no support. But if you don't get nay support with the paid (Pro) version either, then why pay for it in the first place?