For Adblock Plus add a subscription filter called "Malware Protection" even majority of viruses were Windows any case you may stumble at least a backup protection is there.
Well for on demand scanners: Avast for Linux is fine and most of the signatures does included were Windows viruses detection.
Firewall set up for PCLinuxOS: http://www.pclinuxos.com/forum/index.php?topic=98997.0