It isn't entirely clear exactly where the Summary is that you are talking about.
The logon for the forums is validated against your avast account, the my.avast.com is your avast account, your email is the user name and password (what you have chosen). This differs from what you see displayed in the forums 'nightshade', this is your Display Name the username: is still your email, but that isn't displayed.
The name displayed in the forum can be changed in your Forum Profile section, in the Account Settings. But as I said, I'm not entirely sure this is what you are looking for.