I've been running SP3 for a long time, haven't noticed any bugs with avast. What makes me curious though, is whenever I installed the latest SP3 it didn't ask me to validate Windows, before or after, and when I upgraded Vista to SP1 same thing, no validation required
Also one big feature of SP3 is being able to install without a product key if you choose to install without using a product key, you will get 30 days to activate, similar to Vista.