I (and the company I work for) use Grisoft's AVG Antivirus Free Edition. We've found it pretty good so far. It's discovered a few viruses that have arrived at work via email. Only thing is, it doesn't seem to get them when they arrive (like Norton does), it discovers them later, after they've delivered their payload I guess. However it has prevented infection and has successfully removed them. The good thing is that it's free (beer). They bring out virus updates about once a week.

We used to use McAfee Total Virus Defense but it Blue screened some of our PCs and with each new virus update more PCs would be affected so we dumped it. I've talked to other people who've dumped it for the same reason.

Hope this has helped.