Microsoft Security Essentials (MSE) is a simple to use, real-time anti-malware (i.e.: viruses, spyware, etc.) protection software that is available for free. MSE was designed to replaces Windows Live OneCare, a commercial subscription-based antivirus service and the free Windows Defender, which only protected users from adware and spyware. MSE can run on Windows XP, and above (here is the list of minimum system requirements).
Other free anti-malware programs: