Of the freeware antiviruses, I haven't used AVG for a long time. The last I used it was back in 2007 and I was tired of it becoming bloatware. Every update it would use more resources.
I personally recommend Avira Antivir Antivirus. I have been using it for just under 2 years and haven't had a single infection.
Free antivirus - Avira AntiVir
Download AntiVir Personal 9.0.0.407 - FileHippo.com
My colleagues and I all agree that the best pay-for security suite is Kaspersky.
Anti-Virus Software with Anti-Spyware Protection - Kaspersky Lab