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You shouldn’t need to de-register it as such - the format and start from scratch shouldn’t leave any tract of the previous installation. You could however, logon to your my.avast.com account and remove the device.

You should save a copy of your avast license for use with the reinstalled avast version or get it from here - Resend avast License http://www.avast.com/resend-license-paid.php, you will need to enter the email that you used when purchasing the license, e.g. the one that the original email with the link was sent.