I'd recommend the
Zone Alarm with Antivirus package. It's _not_ a freebie, but it's not exactly painfully expensive: about £20 for the first year and about £13.50 to renew annually thereafter.
It leaves out all the bells & whistles stuff: it's just Zone Alarm's firewall (not sure if it's comparable with the Zone Alarm Pro one, but it's certainly considerably better featured than the free one) and an antivirus solution from Computer Associates: effective and lightweight. I don't even bother turning it off for games: it just doesn't make enough difference. Slows things up a little when it's actually scanning, but not that much, and that's to be expected.
I've been using it since it came out, about 2 years ago. it's _never_ given me any of the problems I've had with other AV.