I use ZoneAlarm Free antivirus and firewall, with 2 way firewall installed. It's pretty good, it runs off of the Kaspersky engine (I thinks) so it uses very low CPU + little memory but is great at finding and detecting viruses !
The only bad thing about it is that the two way firewall is a bitch to configure for the first 2 weeks you use it. You keep getting pop ups every time you start an application like "Do you wish to grant the application XXXXXXXX permission to run / use outgoing connections?"