avast! is being tested on new Windows builds continuously (just a few week ago, Vlk has been to Redmond to check avast! compatibility with XP SP3, Vista SP1, etc.) and necessary updates, if any, are implemented. When these new Windows service packs are released, avast! (the current build at that time) should already be fully compatible with it. So, if you keep your avast! installation up-to-date, you don't have to worry about it.
(Though I think you'll have to wait a bit more for XP SP3 itself...)