Antivirus software is essential for safe internet browsing.

I use Avast 5. I had version 4 installed on my old PC for several years. It's a good free antivirus. I also use Comodo Firewall 2.4, which is also free. From a security standpoint, it's very useful to use the Firefox browser with the NoScript extension: it protects against script-based attacks that an antivirus might not detect.
I also have Malwarebytes Anti-Malware and the AVZ utility.
In addition, I sometimes download DrWeb CureIt. I've also used Kaspersky AVP tool.