A free version should provide adequate protection, but the purchased software will be the best option.
Have tried several of the free versions of antivirus and a few paid versions of anti virus software as well...
With the free version of AVG, my computer got a virus even with everything up to date. AVG would see the virus but would not touch it.
On another PC I had a paid version of Mcafee. Same scenario, got a virus with the program being up to date. Had to shell out more money for Norton of all programs to get that PC cleaned..
And while on the subject of Norton, it's an infection in its own right. Its extremely bloated, attatches to other files and makes for a hell of a time to completely rid the system of its bits if you choose to go with another program.
Computer Associates, and Avast I have also ran into problems with viruses.
So far the only free program I have not encountered a problem with is Microsoft's Security Essentials. BUT I only have it on my laptop at the moment and don't use the laptop much these days..