Trend Micro Anti Virus is probably the best AV out there that Ive used and 40$ a year.
It detects everything you throw at it (have had it since 2002) and its lightweight. In addition, it monitors your activity to learn when the best time to scan. Then every week generates a report of what its detected, viruses, spyware, malware, etc. Spybot S&D is a great anti spyware, used to use it back in the day of XP when you could easily get spy/ad ware from anywhere.
Nortons good if you never plan on doing more than Word, Excel, surfing the web since it uses 99% of your CPU when it scans.