NOD32 is the one!! I used AVG free's for years. It's very good for free but has fallen over the past year. Kaspersky has the best signature file for on-demand runs. But NOD32 has made major improvements and that together with it's unrivaled heuristics for catching threatware before signatures are even written. And there is no AV that has less of a system performance impact without sacrificing safety. Whatever you do, stay away from Norton and Symantec.
Use NOD32 and use Trend's online HouseCall as a weekly/monthly backup-on demand.