According to an article by the Washington Post,
Norton Antivirus had one of the worst average response times
for providing virus definition updates based on the worst virus outbreaks,
lagging behind every major competitor, including NOD32, Kaspersky,
Quick Heal, F-Secure, Sophos, Avira, Trend Micro, F-Prot, Panda, Protector Plus, AVG, avast!, ClamWin, and McAfee.
Avast is a very effective and free anti-virus program because it can do
a boot time scan in safe mode that will remove all those stubborn trojan
viruses that hide inside the system restore folder.
I would suggest that you get Avast Home Edition version 4.8.
It now protects against spyware, rootkits, and other forms of nonvirus malicious software.
Best of all, it's free for personal use.
Avast Anti-virus installation
http://www.avast.com/eng/avast_4_home.html
How to configure Avast ?
http://www.infodiv.unimelb.edu.au/soe/antivirus/avast4home.pdf
Avast have very good detection rates for virus
http://www.virusbtn.com/Session-43ed5f077cb4ad87036a9f509c0a256b/news/2008/09_02