Hi...
This exact problem happens for me as well, but interestingly enough when i reboot i find that avast is still starting up but it is switched off...
I have to restart manually through administrative tools/services as the Avs4 control service has been disabled in startup & sometimes Avast anti-virus too.
I have virtually the same specs working on winxp sp2 , it is a minor annoyance that is easily sorted by restart & the error shutdown message doesnt seem to effect shutdown too much, has it got something to do with the auto-update not shutting down..?
(still a great piece of software tho
)