But are Avast updates going to help with the new XP weaknesses?
No one can say what the future holds, but avast will continue to offer protection wherever possible. But the user also has to take some responsibility for their protection also (as I have mentioned earlier) or you are forced to upgrade your OS.
If now my risk of being infected is, let's say, 1%, how much is it going to be afterwards (more or less)?
Again no one can say in terms of percentage risk, some people, because of poor security practices are more prone to malware than others. We don't know your security practices or what proactive measures you might already have in place over and above what security applications you have installed.
In all honesty I don't think it will be as bad as the doom-sayers are suggesting, after all there are people still using XP SP2, having elected not to install SP3. This means that security updates stopped some time ago as they didn't have SP3.
But as I keep banging on, users have to take some responsibility too. If you don't want to lose it back it up, have a recovery strategy and apply it regularly.