Microsoft Security Essentials

Microsoft Security Essentials is the official Antivirus from Microsoft.
Microsoft Security Essentials keep your PC Online and Offline, viruses, or spyware.

Antivirus Microsoft Security Essentials is free, and designed to be very simple, so it is very easy to use.
Microsoft Security Essentials can be run in the background and efficient.
Updates are also very easy.

So you can use Microsoft Security Essentials, you should use windows genuin from Microsoft.
To get Microsoft Security Essentials, please visit the official link from Microsoft here