Because of the wide range of answers you're sure to get, I'd recommend doing some research and trying out a few different products (ONE AT A TIME) before settling on a final decision.
The most respected Antivirus Testing Companies are AV-Comparitives, AV-Test and Virus Bulletin. Check out a couple of the top-rated programs and buy/use the one that best fits your needs.
Personally, I've used ESET NOD32 Antivirus since early 2006 and have loved it. It's worked great with every Windows and Mac computer I've had and I'd highly recommend it. It's very light on my system, has saved me many times over the last 6 years and they even offer free Technical Support to their customers, including (should you ever need it) malware removal assistance. I have enjoyed it so much that when I found out they had an office in San Diego, I got a job there.