I get asked all the time what antivirus programs I recommend. All PCs should have an active antivirus installed. Unfortunately these days computers are attacked and at risk of being infected no matter how cautious you are online.
The people who deploy viruses and malware do so to collect money from replacing ads on your computer, collecting a ransom by encrypting your files or by using your computers resources. The best line of defense against these infections is a good antivirus.
Windows 8 now comes with an antivirus pre-installed. This is a good start but it’s far from perfect. What you need is a standalone tool to thwart attacks.