Many people are getting new computers for Christmas, and the concern about antivirus always comes up. New computers typically come with a trial version of some antivirus or internet security software. Unfortunately, many times people allow these to expire and never purchase or install something else.
There are lots of good free antivirus programs out there, so no reason not to have one installed. Please be aware, if you are going to install a free program, you should first uninstall the trial version that came with your computer.
If you are using Windows 7 (or previous) I would suggest Microsoft Security Essentials -- it is a good, lightweight antivirus, and does a decent job of protecting your computer, all for the low cost of $0.
If you are using Windows 8, please be aware that Microsoft Security Essentials has been replaced with the built-in Windows Defender. It is my opinion that Windows Defender alone is not sufficient for most people. For Windows 8 users, I would suggest the free AVG Antivirus Free 2014 program.
For all PC users, I would also recommend the following 2 programs to help with computer security.
CCleaner -- is a utility to remove temp files and other unnecessary files. Lots of malware place themselves in temporary directories, so this program indirectly helps to keep your computer safe by removing them.
Malwarebytes Anti-Malware -- is a great anti-spyware program, and does an excellent job of finding and removing files that may compromise the security of your computer. The free version works just fine, but note that during install it will ask if you want to start a trial of their pay-version (if you don't make sure to select appropriately).
Using MSE or AVG antivirus along with CCleaner and MBAM will help keep your computer safe and clean. For most people, this combination will suffice. For people who are using this and still find themselves getting compromised or infected, I then usually recommend Kaspersky (which is a paid product).
Note that you should not use file-sharing programs such as Limewire, Frostwire, or Bittorrent -- many shared files contain malware infections. (Lots of computer repair jobs I do are caused by this).