I had probs with the 'Renew now' link as well, I finally renewed successfully by going to:
http://www.avast.com/registration-free-antivirus.php#register-form I entered name & email addy (the original one that my expired Avast was registered with) to send new key to.
Received email: (although it says Avast 5 it worked ok for Avast 6)
Dear avast! user,Your registration of avast! free antivirus was successful. Your license key must now be inserted into the program.Your license key is:---------- cut here ----------WXXXXXXXXXXXXX-ASXXXXXXX---------- cut here –
If you are using avast! Free Antivirus v5.0, you will see an orange ball icon.** For users of avast! Free Antivirus v5.0 (orange ball) ** 1. Highlight the license key above, then right click on it and select "Copy" 2. Right click the orange "a-ball" in the bottom right corner of the screen, then left click "Registration information" 3. Click the "Insert license key" button at the bottom of the screen. 4. Right click in the empty Registration box then select "Paste" 5. Click "OK". Your license key is now inserted and the program can now be used free of charge for a further 12 months.
Job Done :)