I have worked on repairing computers for the last 6 years and have cleaned more viruses and spyware from computers "protected" by AVG and other free solutions to know that its best to purchase good software.
I suggest Norton Internet Security, Not Norton 360. I also like Kapersky and Trend Micro Anti Virus.
Also note that It depends how you use your computer and the sites you visit that determine if your system get infected. Video gaming cheat sites, online gambling and poker sites, Adult sites are a major problem for malware and P2P software like Kazaa, WinMX, Limewire, etc. mainline viruses directly into your system.
If your like most people and do normal surfing, online purchasing from respectable vendors and stick to the best known car sites, you won't have a problem.
Having said that, having free antivirus software is better than none.