As said, AVAST or AVG will both do you fine.
One year AVAST beats out AVG in detection scores and the next year AVG comes out ahead. Both are free.
One caution, in the past year or so, both have become somewhat adapt at trying to push things into the installation, such as toolbars, Google Chrome, etc.
LOOK carefully at each pop-up screen as you are installing -- LOOK at everything....and make sure you are not installing any extras that you really do not want.
AVG was "worse" on that point IIRC.