How long ago did you have this account and by ‘this site’ I take that you mean forum.avast.com ?

The old avast forum had accounts that were part of the complete forum package; you could change the associated email address in your forum profile. Now you can only change your email address in the my.avast.com.

Now your forum account is linked to your my.avast.com account. You can click login in the forum but the email address and password are validated against id.avast.com.