Could it be system specific since I have never experienced this problem in nearly 2 years of using avast. And I really tinker with my system..
I have recently installed it on 3 systems (one desktop and two laptops). The two laptops are used regularly, and I get the occasional error message when shutting down. One machine is quite loaded with software, the other one has been clean installed with only Office 2003, Spybot S&D, Ad-Aware and some other small applications (like Adobe Acrobat Reader). Both laptops have SP2 and the latest Windows Updates.
Oh, small Edit here: both laptops are connected on a WiFi network behind a Firewall and on a cable connection to the Net. I can't remember seeing the error on the desktop that goes straight into the router, but I don't boot that unit a lot, and the error is intermittent on the laptops anyway...