Microsoft Security Essentials
Microsoft Security Essentials (MSE) is a free antivirus program created by Microsoft. It provides real-time protection against viruses, spyware, rootlets, and trojans. It can be installed on Windows XP, Vista, and 7. Windows 8 does not support MSE as it features a built-in antivirus program called Windows Defender (which is very similar to MSE and uses the same virus definitions). MSE can be downloaded and installed through Microsoft Update or directly from Microsoft's website.
MSE can be downloaded here.