Antivirus as the name suggest is a software tool which help us to combat malicious viruses, adwares, malwares, spywares which can potentially harm your system. This harm may be cause in the form of modifying some files to the destruction of whole Operation System.
Linux is safe in these terms as virus does not effect the Linux OS as it is saved in the form of a text file in upper memory band. But windows OS is more vulnerable to viruses as they can directly alter the behavior of OS.
Two free but best and recommended Antivirus for download are:
Just download any of these antivirus packages to have a clean virus free PC.