There are several good and free anti-virus programs or software for PC/Windows users. I personally don't like some of the "paid" security systems out there. Even before the free or trial periods that come with most computers is up or even started, I usually remove those security systems. I keep the Windows Security Center of course, but the others (usually two well known ones) I remove. They usually have a time period where they are free, but then later want to charge you. One in particular will bug you to death to buy its service, even after you have removed it. I normally run AVG anti-virus. It's free and works pretty good. There are other free anti-virus software out there and I have tried one or two, but I always come back to AVG. For me, it's pretty user friendly. I can run a whole computer scan or select just certain scans that only scan drive C, which saves a lot of time. AVG also has a link scanner, that scans links for security risks. Before, I would rarely used that part of the AVG free service and would disable it, but now I am using it again. For my money, or more correctly, for my money I choose not to part with, the best free anti-virus protection is AVG and I highly recommend it. I personally like it even more than the one's you have to pay for.
AVG Site Link