When it comes to internet security, the matter is not about if some thing is paid or free, but about how the job is done and how good its done and at what cost.
There are notably 5 free antivirus solutions that come to my mind when it comes to my mind. Microsoft Security Essentials, Avast, Avira, BidDefender and Comodo. Among these I recommend the first, Microsoft Security Essentials. Its light weight, bundled with Internet Explorer 9 and Windows Firewall you got a great solution protecting your computer.
When you come to the paid areana Norton, Kaspersky and McAfee are the big sharks. All these software add up considerable drag (slows down) to Windows and have a harder learning curve with respect to MSE.
I will include 2 links, the first is about a short description about the 5 antivirus I talked about. The latter is of some of the features MSE has.
If you are going free, I recommend MSE which is as good as any paid software. If you have issues getting things done free, well Kaspersky is at your service.It has some cool features and a great detection engine.