The Final version of Avast Antivirus 5 (5.0.178.0) is not yet official available at  Avast website, but you can download Avast Antivirus 5 free version from google pack.

Google replaced outdated Spyware Doctor and Norton Security Scan with Avast Antivirus 5.0 , which has gone under a complete overhaul. Although Avast Antivirus 5.0 is not yet available in English version of Google Pack, but you can download it by changing  the google pack  language as shown in above image, beacuse Avast will be installed in English (check below second screen shot ).

Currently you can see Avast Antivirus 5 in google pack for the following languages: French, German, Italian, Czech, Spanish, Russian, Portuguese, and Polish.

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About Avast Antivirus 5
Avast 5 brings important changes to it’s visual appearance and also made it lighter on system resources . Another major improvement is it’s new behavioral shield, which monitors your system in real time for threats, that is not available in Avast antivirus 4.8.

Also AV-Comparatives.org ranked Avast, and Microsoft Security Essentials as the best free antivirus software’s of the year 2009.

Download Avast Antivirus 5
Visit this link http://pack.google.com/intl/de/pack_installer.html?hl=de, click on “Jetzt herunterladen”,then click “zustimmen um Download” and continue with google pack installer.

Note:
The final version of Avast Antivirus 5 will be available very soon (mostly tomorrow) @ Avast website, so check this link
http://www.avast.com/eng/download-avast-home.html

Update: Avast Antivirus 5 now official available at Avast website, so download it from above link.