IMHO I would say it is NOT a Avast problem since Avast is beautifully working without sp2. So the thing that changed is Windows (sp2), not Avast. So blame Microsoft and not a working product like Avast. Besides as MS stated "DO NOT RUN SP2 IN A OPERATIONAL ENVIRONMENT"
And as Vlk said: Avast is working fine with sp2, so it is more likely YOUR system that causes the problems.