@kevinpang, for XP you should be using at least SP3 and have Windows validated / registered before installing / updating Avast.

This is NOT a strict condition (specially for previous versions of Avast), and it is NOT a “solution” to the problem, but it seems that some users are having this type of problem with the latest updates of Avast, and the “workaround” might help you.

In any case (independently of Avast), it is recommended to have XP SP3 installed if you still haven’t.