Installation is quick and easy (though beware, it'll install the ZoneAlarm browser toolbar unless you choose the Custom Install option and decide otherwise). And the package does its best to avoid conflicts with other antivirus packages. So if you have one installed already then ZoneAlarm Free Antivirus + Firewall will detect this and turn off its real-time protection to avoid clashes.
Otherwise, though, there are plenty of scanning options: you get multiple scan modes (Quick, Full, Full with Archive files), a versatile scheduler, and a vast array of settings to tweak (there are 22 "Behavioural Scan Options" alone).
And as a bonus there's also basic phishing and identity protection, with 5GB of online backup space thrown in for good measure.
Download: ZoneAlarm Free Antivirus + Firewall v12.0.121.000