Microsoft Security Essentials

this is microsoft antivirus software, which is absolutely free as long as the os is genuis,
microsoft security essentials differert editions as follows

Windows XP(32 bit)：
Microsoft Security Essentials 1.0.1611.0 for Windows XP 32-bit

Windows Vista、Windows 7(32bit)：
Microsoft Security Essentials 1.0.1611.0 for Windows Vista/WIN7 32-bit

Windows Vista、Windows 7(64bit)：
Microsoft Security Essentials 1.0.1611.0 for Windows Vista/WIN7 64-bit

this is “Microsoft Security Essentials" website.
http://www.microsoft.com/security_essentials/default.aspx?mkt=en-us