What about the original email you received with the license file?

http://www.avast.com/contact-form.php?loadStyles

You should use the original email address that you used to register in first place, or at least mention it.

The sales email address is also a possible contact method, but you should provide as many details as you can (specially the original email address for your original registration) so they can really resend you the license.

You could also add the link to this same topic, just in case.