Best Answer: If you want a free anti-virus I recommend in order of popularity, but please choose the right one for you (install only one):
1. AVG Free
a. A simple interface (easier to use than Avast! and Avira)
a. Real-time protection
b. It has an option to delete a virus once found
c. Can detect about 87.46% of viruses.
Cons: Too many icons and confusing user interface
I advise against Norton anti-virus, it's bloated software that takes control of your computer. The best anti-virus available is Kaspersky or NOD32 (recent tests at av-comparatives.org).