First among freeware av's : Avast or AVG !? 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 endup updownloading 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.
Now as to the freeware av's drawbacks :
AVG and Avast have poor detection of polymorphic and 0-day malware in addition to having limited self protection.
The free Avira lacks an email scanner; that's only available in the paid version. The self protection is also poor - it can be terminated with Windows Task manager. The free version also constantly nags you to upgrade.
The Kaspersky-based AOL AVs lacks the html scanner found in the full Kaspersky product and consequently doesn't offer the same level of protection against hostile sites. It's also quite heavy on resources and is not suitable for slower PCs. AOL AVS can be terminated by a hostile agent as well
BitDefender lacks a real time monitor as well as email scanning.