Microsoft Security Essentials
Microsoft has released their new Microsoft Security Essentials application, which supersedes Windows Defender and also includes an anti-virus component. If you use Windows, I would recommend dumping whatever anti-virus program you’re currently using, and installing MSE instead. It’s free, and coming directly from Microsoft, it should do a better job of protecting Windows without accidentally killing Windows, as third-party AV products have done occasionally. And many of those third-party AV apps have become huge bloated monstrosities over time, whereas MSE is small and quick.
You can check out Ars Technica’s writeup for more details.
Note that it’s a bad idea to have multiple AV products installed at the same time, so you will need to uninstall your existing one first. But MSE does do a Windows validation check before installing, so before uninstalling your existing AV app, you should launch the MSE installer and go through the first few screens to make sure your Windows installation passes the validation; then cancel the installation, uninstall your current AV app, and finally re-start the MSE installer.