If u don't have any installed,u could start here...
U can use Avast
along with Malwarebytes
and Comodo Firewall
that is free, but got great results in defending your PC. And u can configure it to your satisfaction with ease.Avast! -- Free Antivirus Malwarebytes Anti-Malware -- helps you defeat Malware once and for all SUPERAntiSpyware -- Advanced Detection and Removal Comodo Internet Security -- The best free Firewall