This has always been the never-ending debate among these two major freeware
av. With regards to the PROTECTION here are my observations :

Avast in my opinion, is MUCH better at detecting virus files inside
compressed (zip'd) archives. Even AVG's complete scan will NOT detect ALL
infections inside the Zip files. And if the compressed files are .rar or
other formats, AVG's odds of detection are even less.

Next : Detection of email borne viruses in AVG is at best average. If you
have Outlook Express 6 client as part of Windows XP, you could easily end up
downloading and saving some common email borne viruses. Its only at the time
of extraction or saving the extracted infection that AVG gives a warning !
AVG even with using the Outlook Express plugin, u could download the virus to
your machine as an eml file, and it wont even warn when you open the eml
file.

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http://www.WinVistaClub.com
Microsoft MVP 2007 Awardee.