Not just for Free.People always install a anti software with their pc which can protect your pc be safe from Internet virus,i think no any anti-software can 100% to sure that you will never be attacked by hacker,sometimes anti-software like a  humanvegetable that means a new virus can take control your pc without your consent.



If you only install one anti-software ,the % of attacked will be rised,before i told that no any anti-software can perfectly work their job to fight down all of the virus.

Suggestion:

you can install a anti software,and regular scan your system.

Try different anti-company product to find your perfect match.

Using online scanner to scan you PC.why i suggestted you to do that.i told it before,no any software can be perfect, Exactly shortcoming is patent,so if you try different scanner that means you can lower down mistakes and  veracity.

In this section,i will introduced some online Free scanner.


Eset

Nod32 Eset http://www.eset.com/online-scanner
How to use it 

http://www.f-secure.com/en_EMEA/security/tools/online-scanner/

http://quickscan.bitdefender.com/ (IE is suggestted) 60-Second QuickScan