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Ok, sounds like you do not have Avast! Free, but instead a free trial of an Avast! paid product, either Avast! Pro or Internet Security.
From there, seems like you filled out the online registration forum for Avast! Free and got sent the code, which of course would work fine for registering Avast! Free for a year, but wont do much to activate a trial of one of the paid versions, since no $$$ has changed hands, and the paid versions need a license file you receive via email after you buy.
So, if you want Avast! Free for a year, simply uninstall whatever free trial you have, install Avast! Free, and the registration you have already should work fine for that. But to activate the Avast! you currently have installed, you will need to open your wallet 