The best free antivirus program for Windows WAS Active Virus Shield by AOL. It was better at protecting against viruses than even popular commercial antivirus options such as McAfee Virusscan and Norton Antivirus. That’s because it used the same underlying software used by Kaspersky, makers of one of the most effective commercial antivirus programs. Well, last week with no warning AOL discontinued their contract with Kaspersky, shut down their own update servers, and users of Active Virus Shield were no longer receiving daily antivirus updates. There is a simple hack to continue receiving updates directly from Kaspersky instead of AOL. But it may be best to stick with software you know the community can keep supporting even if a company stops doing so. ClamWin is the most popular open source antivirus software for Windows. There is also a Mac version (ClamXav) and a Linux version (ClamAV).