BarokOnline

Four things I need to point out:

• Comprehensive malware protection
• Simple, free download*
• Automatic updates
• Easy to use

I don't have problem so far using this great software for more than 3 months already. And not only that. I have been running my PC without any other anti-virus, anti-malware or other anti-malicious stuff running on my PC.

Visit their website for more info regarding this: MS Security Essentials.

Here are the basic hardware requirements you might consider before trying this out:
Operating System: Genuine Windows XP (Service Pack 2 or Service Pack 3); Windows Vista (Gold, Service Pack 1, or Service Pack 2); Windows 7

• 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.
• 140 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.
• Internet Browser:
  • Windows Internet Explorer 6.0 or later.
  • Mozilla Firefox 2.0 or later.
• Microsoft Security Essentials also supports Windows XP Mode in Windows 7. For more information, see the system requirements for Windows XP Mode in Windows 7