"The web sites are mostly offering keys to our free Home Edition product, but not in all cases, though charging users as though it were for our paid-for Professional Edition, or, when email "invoices" are received by customers, there is no mention of avast! Instead a list of programs is usually offered that the customer has never heard of, let alone agreed to buy."ALWIL Software has provided a list of site names that are not authorized resellers of their product. See Fraudulent sales of avast! products.
To select and purchase a licensed copy of avast! go to the Desktop Solutions page.
Note: avast! 4 Home Edition is available free for personal use.
via DP's Security Bits