I've had pretty bad experience with AdAware... It was on our University laptops, but they recommended we uninstalled it, because it was giving many people issues with glitches and viruses.
AVG is free, and a decent anti-virus program. However, I've been running no anti-virus software for 3 years now (for two main reasons) and I haven't had a virus since.
Norton was actually the cause of a virus I got back a few years ago. A virus had attacked Norton first, then the rest of my computer. If I hadn't been running Norton, I wouldn't have had the virus.