I've always used the free version of Avast anti-virus and have never had an issue with any sort of viruses or performance issues. Though, I did buy an extension of Avast at one point to clear up old files and other random programs that were tedious enough in removing to spend around 7 USD. However, I think some anti-virus software purchases could be beneficial if you feel you download/browse through a lot of (potentially) risky files and sites. Otherwise, much protection is done through personal awareness and prevention.