I use avast!, even though it seems to have a propensity for triggering false positives quite a bit. This has only happened once in the past year and a half (a couple of weeks ago when it flagged an Adobe component as a virus), but early last year it seemed every month or so some innocuous file would be flagged, throwing me into a tizzy.
Honestly...avast! flagged the Windows Movie Maker installer as a trojan at one point last year. The Windows Movie Maker installer that came with Windows, mind you.