Comodo Internet Security is a magnificent and professional security application; it is also totally free. You could consider it as an antivirus as its mission is to protect the main systen files from any viral attack. This way it will always keep your PC working and stable.
The application works discretely from the Windows taskbar, and is always active and alert for the slightest attempt by a virus to attack your PC. From the Comodo Internet Security interface you can configure the firewall so that the user can decide which ports are accesible and which not.
Apart from viruses, Comodo Internet Security also detects spyware intrusion, and also protects and blocks the Windows Registry from modifications attempted by viruses.
Kaspersky Internet Security 6.0.2.621 ( 2008-12-31 ) One of the best antiviruses that exist, that detects worms, trojans etc, in addition to being an antispyware, antispam and firewall program.