No problem here, WSC on XP SP3 recognised avast directly after update and reboot.
No RPC errors either, clean as a whistle, apart from it being a large update 6.8MB (I believe) going from beta to standard version, no fun on dial-up.
Interestingly Microsoft's Auto Update kicked in during the avast update, which I interrupted when I disconnected to reboot after the avast update, chancing my luck perhaps, thankfully no issues there either.