I've recommended AVG Anti-Virus before but now there is a better and even cheaper free version
AVG Free 8.0 has been released.
According to the company, the software is capable of protecting you from viruses and spyware and also provides a security toolbar to help make your searches safer.
The paid-for version of AVG's 8.0 anti-virus tools have been available since February.
The full versions include LinkScanner technology that will help protect against malware that attempts to infect your PC via compromised websites.
"With the release of AVG Free 8.0, we are underscoring our belief that all computer users, regardless of their computer usage needs, have the right to a safe and worry-free computing experience," said Karel Obluk, chief technology officer at AVG Technologies.
If you haven't tried it already then try it here:
http://free.grisoft.com/