There are many different types of free Antiviruses available for your download on the net, from personal use and trial and error I have found these two to be the most worth while.

• Avira AntiVir (Free Edition) - Free.grisoft.com
• AVG (Free Edition) - Free-av.com/

Both of the above are what I personally use on my Desktop and Laptop and I find these to be extremely efficient at keeping me Virus free. Both Antiviruses are very easy to configure and also they do not use as much system resource as some of the popular paid for services which hog your RAM and can even slow down your machine.