If you have used the Avast forums, and the password you chose was unique to that system, then there's no need to do anything. However, if you were registered on the forums and you used the same password elsewhere, on other systems, it would be wise to change that password on those other systems. Otherwise, if someone does manage to discover the Avast usernames and passwords, they can then be used on other systems too.
http://www.techsupportalert.com/content/if-you-use-avast-forum-you-need-change-your-passwords.htm
http://www.techsupportalert.com/content/if-you-use-avast-forum-you-need-change-your-passwords.htm