Can't say anything about avast as I've never used it. Personally I use AVG 7 Pro which has proved extremely effective and the updates are, in the main, at least daily. I also have Kaspersky (also very effective) which doesn't run at the same time but which I keep for occasional scans - a kind of second opinion. The only reason I don't use Kaspersky in the background is that it's much more resource heavy than AVG 7. Do NOT have both running in the background at the same time. Same goes for software firewalls
For firewall my preference is for Zone Alarm Pro
With internet security being what it is I'd recommend putting your hand in your pocket for the Pro versions as they're much more fully featured
Steve