I'm afraid Avast won't work properly on XP SP2 anymore - not even the old versions that worked previously. The thing is that XP SP2 isn't able to verify new SHA2 certificates - so most of the online services (such as reputation services, and subsequently DeepScreen) and even parts of the program UI (that are actually fetched from our servers) won't work. I'm afraid that even if we wanted to keep obsolete certificates on our servers (which we don't), we couldn't - because the old certificates have expired or will expire soon, and it would be tricky to find someone to give us a new certificates with a complete SHA1 chain.