There are quite a number of website on the net where you can get this . The more popular one is like Antivir , Avg ( I'n using this) . I would recommend AVG because it's effective and easy to use. I've tried the frree version og Antivir but there's one time when I copy and paste some file from a DVD -rom when one of the file contain a few type of this PARITE VIRUS . AntiVir system suddenly became havoc and it itself became a virus . Sound hard to believe ? It took me a couple of hours to delete and /or quarantine all the Parite virus using AVG Home Edition downloaded from www.grisoft.com .It's free for home user.
Since then I choose AVG because it's more stable and effective .You don't have to pay for expensive Anti-virus out there if you're home user.Have AVG Home Edition is good enough.