I use free Avast antivirus + free Malwarebytes antimalware.
It is good to have both solutions. Antivirus looks for viruses in real time. Antimalware scans on demand for non-viruses like the one I got recently - multiplug - http://pcspywareshield.com/guides/multiplug/
Single antivirus cannot do all the work. Malwarebytes is focused on malware-adware - which is not a virus and gets on to you PC most often via bundled downloads of media fiels of free soft