Try logging on to your my.avast.com account, the avast forum is linked to that now. once logged on to your my.avast.com account the forum should hopefully recognise you.

That is what I did first time round on this new identification/validation method.

Your avast forum logon uses the email address from your my.avast.com account as the user name for the forum.

For the first three posts (if you have created this as a new account to try and get on to the forums) the captcha is used as an anti spam measure so it has to be relatively hard to beat the bots.