Microsoft Security Essentials

Unknown to most people, Microsoft has released their own anti-virus/anti-spyware program. 

Microsoft Security Essentials is available completely free of charge to anyone with a valid copy of Windows XP or newer Microsoft operating system.

Tests have shown MSE (Microsoft Security Essentials) as being effective at eliminating and preventing viruses and spyware. MSE has a very little impact on system performance, even old systems function well without experiencing any noticeable decrease in performance. Best of all with MSE you don't run into some common problems and conflicts caused by some of the big-name security programs. 

Microsoft Security Essentials is excellent for any home and small business users. I highly recommend it.