Velocity Reviews have a roundup of the most popular free Anti-Virus applications, which provide excellent protection for different types of users:
"Although it isn’t possible to fully protect your computer from a huge array of viral, spyware, and other malware threats, a good anti-virus application goes a long way to securing your PC. Now that most computers are permanently connected to the Internet, it is more important than ever to ensure you have an up-to-date and reliable anti-virus package on your computer. Some people still don’t have any installed, thinking that because they are careful what they download, they will be safe. Others don’t like the thought of losing even the slightest amount of performance from their PC. However, the importance of getting a good virus scanner cannot be underestimated – and this lesson is often only learned after a virus infection."