A quote from Wikipedia that helps explain why, personally, I think that Avast is superior to AVG.
AVG Anti-Virus Free Edition
According to AVG Technologies, over 70 million users have AVG Anti-Virus protection, including users of the Free Edition. On April 24, 2008, AVG released AVG Anti-Virus Free Edition 8.0. Along with a more colorful interface, the new program added some new features - a combined Anti-Virus and Anti-Spyware protection engine, LinkScanner Search-Shield safe search protection, plus the AVG Security Toolbar - which were previously only available in AVG commercial versions.
There are some limitations with AVG Anti-Virus Free Edition 8.0 compared to the commercial versions of AVG products. These limitations include:
* Less protection - AVG Anti-Virus Free Edition 8.0 provides only base level protection against viruses, spyware and safe searching. It does not provide safe surfing (drive-by download protection) nor protection from hackers, keyloggers, spam, phishing attacks and malicious file downloads that can come through instant messaging and attachments from seemingly friendly sources. This protection is only available in the commercial versions of AVG products.
* Slower updates - AVG Anti-Virus Free Edition 8.0 receives updates via a lower priority service. Priority updating via high-speed servers is only available for the commercial versions of AVG products.
* No telephone or e-mail technical support - There is no telephone or e-mail technical support provided by AVG for users of AVG Free Edition products anywhere in the world. AVG Free Edition users have access to support via the self-help AVG Free Forum.
* No customisation - Scheduling options in AVG Anti-Virus Free Edition 8.0 are very limited (only one scheduled update per day, one scheduled scan per day etc.). The AVG Resident Shield configuration is limited with no exclusion settings. AVG Anti-Virus Free Edition does not offer advanced testing options, password-protected archives reporting, adjustment of scan process priority and many other features.
* No server support - AVG Anti-Virus Free Edition 8.0 cannot be installed on server operating systems (such as Windows Server 2003), nor can it be used for the scanning of network drives.
AVG Free Edition is responsible for popup ads. These are for the non-free AVG Anti-Virus 8.0 and AVG Internet Security 8.0 products, which claim to provide more comprehensive levels of protection.