These are some of the best, completely free antivirus software and add-ons available on the internet for download.
Avast Home Edition: This is a completely free but one of the best antivirus applications available anywhere, which is intended for non-commercial use. You can download and register it on their website (www.avast.com) free of cost. The Avast Home Edition features an On Demand Scanner, On Access Scanner (to protect e-mail clients, instant messengers, etc). It also has P2P protection and a lightweight firewall. The NNTP Scanner can scan all Usenet Newsgroup traffic and all operations with files on PC. It also has a real-time protection feature which is rather powerful. Program and database updates are also incredibly fast.
602LAN Suite (http://www.software602.com/products/ls/): This secure mail server has an inbuilt antivirus and anti-spam guard. The firewall is meant to control internet sharing and is equipped with NAT and Web content filter proxy. It also features standards based SSL SMTP/POP3 e-mail server that enables encrypted communication, junk mail protection and an integrated fax server. Access to the Web mail client, shared address book, remote administration and user home pages is provided by the integrated Web server. Other features include includes text content based Bayesian spam filter, Web Mail Client with WAP support, network address translation, attachment filter, PureSight proxy content filter, auto-update and ActiveReports.
AntiVir PersonalEdition Classic (http://www.free-av.com/): Another one of the few, but great free antivirus software available for download. The AntiVir PersonalEdition is for individual and private use on a single PC-workstation only, which can detect and remove more than 230,000 known viruses. It features a resident Virus Guard which monitors file movements automatically such as downloading of data from the internet. It scans and repairs macro viruses and protects against boot record viruses and master boot record viruses, Trojans, worms, and other malicious activities. You can easily update the database and support is free of charge through the AntiVir Bulletin Board (http://forum.antivir.de/).
AVG Free Edition: The free edition is available to home users for personal and non-commercial use on a single computer. Though it is limited in features compared to its professional counterpart, the AVG Free Edition does include useful features such as Automatic healing, Reliable scanning engine, Scheduled scanning frequency and Scheduled update frequency (both once a day only), Optional automatic update upon Internet detection, and Options to define file name extensions. There is also no technical support available.
Futureca AntiVirus Outlook: This is a special add-on designed specifically for Microsoft Outlook. It scans all incoming and outgoing mail messages for viruses in real time, and immediately alerts the user if it detects one. It is quite useful if you are a regular Outlook user, and the virus definition database can easily be updated over the internet. You can download it from http://www.futureca.com/downloads.htm.
RAV4ICQ (http://www.ravantivirus.com/): This is a free antivirus program intended to be integrated into ICQ, one of the most popular instant messengers in the world. The program checks for and removes several types of viruses, worms, Trojans and other malwares. It is easily configurable and can scan inside archives and take the desired actions for infected/suspicious files, and creates a log file for later reference.