AVG Anti-Virus Software
I use Norton and I'm happy with it, But if you don't have the funds to spend for virus protection, check out AVG free edition. (donations accepted).
From their website:
AVG Free Edition is the well-known anti-virus protection tool. AVG Free is available free-of-charge to home users for the life of the product! Rapid virus database updates are available for the lifetime of the product, thereby providing the high-level of detection capability that millions of users around the world trust to protect their computers. AVG Free is easy-to-use and will not slow your system down (low system resource requirements).