AntiVir is excellent, highest detection rates among the free software. And I've had better luck with it than the big names like Norton and McAfee as well. I would also advise a HIPS program such as ThreatFire and if you feel the need for a firewall Sunbelt Personal Firewall is free and no popups to bug you. But when used with ThreatFire, you'll have to configure the firewall to allow ThreatFire to run, just right click on the firewall icon down in the tray and go to the configuration, then intrusions, then uncheck the HIPS protection for the firewall. It's not necessary to have that checked because in 30 days it becomes just the firewall without the extras.