Microsoft Security Essentials is the latest antivirus software which i recommend end users to use for their Microsoft Operating Systems. You can Download it for free from Microsoft website:
Once you get to the site, click on "Download Now" button, choose your Operating System, choose where to save the installer, then click on "Open" button to start the installation process. Once install is complete, automatically Microsoft Security Essentials software will update itself. Make sure to restart your system once it's done installing all updates.