I recommend a few different antivirus programs, which work very well and are free, so it is your choice.
avast!Download link:
avast.com/free-antivirus-downloadScreenshot provided by Softpedia!
Avira AntiVir Personal EditionDownload link:
free-av.comScreenshot provided by Softpedia!
Microsoft Security EssentialsDownload link:
Microsoft.com/Security_EssentialsScreenshot provided by Addictive Tips!
Rising AntivirusDownload link:
freerav.comScreenshot provided by Softpedia!