There are a number of free antivirus programs available.
An anti-virus program will remove viruses from your computer and most prevent viruses from being on your computer. Viruses are programs that do damage to your computer instead of helping you do tasks.
A couple of anti-virus programs that I like and use that are free for personal use are Avast, AVG and Microsoft Security Essentials. All of them will clean viruses and also will help prevent your machine from being infected. Avast is available at www.avast.com AVG is available at free.avg.com Microsoft Security Essentials is available at http://windows.microsoft.com/en-us/windows/security-essentials-download
Anti virus programs need their definitions/signatures updated regularly that is used to recognize the viruses. You have an update in each program to do this. Usually on the free copies you need to manually run updates every couple weeks and if you bought the program it will update them automatically. Also make sure you have them scan your PC every week or two.
You can also get anti-virus programs free to use on your cell phones.
You also need to have a anti-spyware program. Spyware programs are programs that get information off your PC or have irritating ads appear in places that in turn collect information. Most anti-virus programs will detect and clean some spyware. I use superantispyware as it does a real good job of removing spyware and works fine with anti-virus programs. Superantispyware is free for personal use at www.superantispyware.com
You should also get the newest version of your anti-virus and anti-spyware programs each year. People who write spyware and viruses are constantly working to find ways around anti-virus programs. In addition they want to hit the most machines possible which is why viruses occur more on Windows PCs than Linux or Macs.