I recently have been trying two different anti-virus softwares. One of them is listed below.
AVG Free Edition is the well-known antivirus 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). Highlights include automatic update functionality, the AVG Resident Shield, which provides real-time protection as files are opened and programs are run, free Virus Database Updates for the lifetime of the product, and AVG Virus Vault for safe handling of infected files.
I have found that it in fact slows your system somewhat, esp. older systems. I was running it on a Pentium II, and the lag is noticeable for users. So it’s not great for older systems. But it runs well on my Pentium 4 systems without too many problems. I like the fact that scanning is constant, it’s on demand, and it’s updated. Overall, much better than the Symantec Crap that used to be installed on one of my systems.
The other I’m trying is Avast. I’m not done evaluating this yet, but will report back. In the meantime, you can read this post, too.