AVG is the way to go.
As for a firewall, I suggest Sygate personal firewall.
Better than zonealarm, more networking functionality, and it doesn't candy coat crap. It's a geek's firewall.
It also notices application hijacking.
I've also noticed the amount of system resources varies. If you're doing a 600k/256k transfer, it can suck up your processor up to a third. If you're not doing anything network wise, it uses 01% of your processing power.