I reviewed the best free antivirus / antispyware tools last fall. The report on which I was basing some conclusions has been updated. Again, AOL Active Virus Shield is the best at stopping viruses among the free options, ahead of AntiVir, Avast, AVG, and ClamWin. Note also that all these free options except ClamWin come out better than Norton and McAfee (the two most popular commercial antivirus tools). If you ask for recommendations at most places though, people are still predominantly recommending AVG or ClamWin. The latter because it is open source. In addition to not stopping as many viruses, AVG is heavily restricted (after a while you have to uninstall it or pay them) and you are not allowed to install it on more than one computer unless you pay. ClamWin doesn’t support real-time scanning – for example scanning files as they are downloaded – has no email protection, and is the worst of the free options at catching viruses.
For antispyware protection, Microsoft Windows Defender or Lavasoft Ad-Aware are still freely available. There isn’t much difference between the two, they are both good, but I personally have been using Windows Defender since it is by the same company that makes the operating system (Microsoft). And ZoneAlarm and Comodo are still available for firewall protection.