Microsoft Security Essentials:-Free antivirus
Microsoft Security Essentials is a free real-time protection for SOHO and SMB PC that guards against viruses, spyware, and other malicious software.
NOte:Your PC must run genuine Windows to install Microsoft Security Essentials.
Supported operating systems: Windows 7, Windows Vista, Windows XP
•For Windows XP, a PC with a CPU clock speed of 500 MHz or higher, and 256 MB RAM or higher.
•For Windows Vista and Windows 7, a PC with a CPU clock speed of 1.0 GHz or higher, and 1 GB RAM or higher.
•VGA display of 800 × 600 or higher.
•200 MB of available hard disk space.
•An Internet connection is required for installation and to download the latest virus and spyware definitions for Microsoft Security Essentials.
There is an article wrote Mathew J. Schwartz says Microsoft Security Essentials Loses AV Certification