Norton haters, accept the fact (even though for you it's still impossible) Norton is one of the best anti-malware solutions avaliable on the market.
THEY DO NOT USE UP ALL OF SYSTEM RESOURCES, IN FACT IT USES THE LEAST MEMORY OUT OF ALL THE MAJOR SECURITY VENDORS.
Norton Internet Security 2009 (NIS 2009) is currently the FASTEST internet security package (at 7mb memory usage only!) (beats kaspersky by 15mb)
NOD32 IS NOT FASTER THAN NORTON, AVIRA IS NOT FASTER THAN NORTON, NO MAJOR INTERNET SECURITY BRAND IS FASTER THAN NIS 2009.
Proof http://www.pcauthority.com.au/Review/127995,norton-internet-security-2009-believe-the-hype.aspx
NIS 2009 has the lowest system usage (therefore fastest) product on the market. With high a detection rate of 92% on par with Kaspersky.
Norton Internet Security 2009 is the Fastest (more proof)
http://www.bastillwork.net/security-software/norton-internet-security-2009.html
http://news.softpedia.com/news/Norton-Internet-Security-2009-to-Come-with-Super-High-Speed-86321.shtml
http://www.pcadvisor.co.uk/news/index.cfm?newsid=104478
AVG provides a low 81% detection rate and it has FALSE positives, that is non-viruses will be detected as viruses.
http://antivirus.about.com/od/antivirussoftwarereviews/gr/avgfree.htm
AVG Free does not contain Anti-Rootkit protection so rootkits may be hidden in your system.
AVG Free does not contain the new Safe Downloads & Instant Messaging protection (Web Shield technology) so it does not screen your downloads and communication for viruses and spyware.
AVG Free does not protect against identity theft, hacker attacks, infected downloads, infected instant-messaging and spam.
AVG Free does not contain a firewall which can protect you from hackers acquiring access to your computer.
(AVG state this themselves, just click the link below and scroll down)
http://free.avg.com/ww-en/download-avg-anti-virus-free-edition
Also answerers must understand, a free anti-virus alone can't fully protect a system.
By the time of answering this, NIS 2010 has just been released. Faster than ever and using new reputation-based technology, this eliminates the old signature-based scanning and opens new potentials for nexgen security.