Microsoft Security Essentials is Microsoft's FREE Antivirus to protect your PC from malicious programs attack viruses, trojans, spyware, rootkits and protection every time. Based on my experience, this is the best antivirus. Microsoft Security Essentials runs quietly and efficiently so you are free to use without interruption or without a long wait. Now the version Microsoft security essential is v2.0.657.0.
Supported OS: Windows 7, Windows Vista, Windows XP
If you are using windows xp, windows xp sp3 is recommended use
according your needs or your OS, X86 or x64 version