ClamWin is free, opensource antivirus software. I've been using it for a couple of years now and have found it to be very reliable. The software stands true to its promise that virus definitions are updated regularly - I notice updates on a daily basis. Virus definitions can be updated automatically or manually via easily configurable options.
With viruses on the rampage these days nobody can afford to be without the protection of some kind of virus detection software. Although there are many options on the market, they are mostly out to make money, and not necessarily make life easier. Some antivirus software will actually become a virus in the respect of carrying adware or reducing your computer's performance down to tortoise speed - which is the type of thing they should be trying to prevent viruses from actually doing!
With ClamWin you can schedule regular virus scans, and integration into outlook helps to protect against virus infected email attachments. The software itself is automatically updated regularly which helps both to protect against viruses which might target ClamWin, and to ensure ClamWin remains capable of detecting the most up-to-date incarnations of viruses. Because this software is in no way commercial it does what it should without a fuss - it is built by people who know what they want out of the software - not by people who want to spread fear in order to increase sales.
I recommend this to any home user looking for good, dependable antivirus software.