Windows Defender

Windows Defender
This is again a free and powerful product from Microsoft. Windows Defender is a free Anti-Virus and protection system.
You can download it here : http://go.microsoft.com/fwlink/?LinkID=70604
Why Windows Defender is better ?
What I found good on Windows defender is that he control Registy settings. So he alerts you if any program tried to add or delet registry keys and have the ability to deny it.
Another nice thing is the software explorer, that will help you to identify running program, Network Connected program, Winsock service provider and startup programs.
Screen Shoot of Software Explorer

Windows Defender have also a list of definition of viruses and can make full scannes for your computer.
So I advice you to use Windows Defender, you need only to have a Windows Genuine (With Pirated windows you can't download it form Microsoft)
See http://go.microsoft.com/fwlink/?LinkID=70604 for more information about Windows Defender