Avast jumped the shark quite a while ago, prompting me to switch to MSE which I've been quite happy with. However, keep in mind that an anti-virus is pretty much the last line of defense- it's there to try to salvage the situation if you've already fucked up. The first line of defense should be limiting attack vectors- NoScript and AdBlock+ are great for blocking the many of the most common infection vectors, and combined with the basic (put less common than it should be) practice of not doing stupid shit and you should never even need to make use of your anti-virus.
Also, regarding Comodo (as several posts have mentioned it), I find the basic firewall functionality to be useful and fairly lightweight, but their whole Defense+/proactive protection is far more trouble than it's worth and should just be switched off. Also, regarding the firewall itself, I wouldn't consider a software firewall all that good for stopping incoming attacks (your router firewall should be handling that), but rather what I find useful is the basic HIPS functionality, and being able to see (and allow/block) any programs attempting to establish external connections.
