Anti virus is a must be installed software for your PC. Many anti virus software are available in market, they are expensive to buy unless they come bundled with your desktop / laptop operating system.
Freely available anti virus software , is an option to use it in your PC. Here , few such anti virus are listed with the features.
AVG free edition
Avg free edition comes with handful of features. Resource consumption is less. If compared with professional / paid edition free edition lacks many features. Free edition comes with small virus scan engine , which many times will not be capable of handling viruses. We can’t complain much because , as a free anti virus, we cant expect more.
Most widely used anti virus software, free online updates, anti virus kernel is based on the certified anti virus engine.It provides features like
* File System Shield — Real-time protection against viruses and other malware threats.
* Mail Shield — E-mail/Microsoft Outlook/Exchange scanning.
* Web Shield — HTTP protection (local transparent proxy). Version 4.8 also allows the blocking of URLs.
* P2P shield — P2P protection.
* IM shield — Instant Messaging protection. (Scanning of files transferred through instant messaging applications)
* Script blocker — Professional version only; scans webpages for malicious scripts.
* Network Shield — Basic protection against well-known network worms. Acts as an Intrusion Detection System.
* Behavior Shield – Reports suspicious behavior by analyzing the behavior of programs
Avira AntiVir Personal is for personal use only and free of charge. Like most antivirus software, it scans disks for malware and also runs as a background process, checking each opened and closed file. It can detect and possibly remove rootkits. It also performs Internet updates (daily by default) in which it opens a window, with an advertisement suggesting that the user purchase Avira AntiVir Premium or Avira Premium Security Suite. Avira updated all its products to version 10.0 in March 2010.
As a free service, Avira offers a bootable rescue CD to all users, regardless of whether they have any installed Avira products (free or non-free), and irrespective of the machine’s operating system. This CD contains its own bootable Linux-based operating system and the AntiVir program with its latest virus database. If a computer cannot be booted, the CD may be used to boot the machine into the AntiVir program, then to scan for and remove detectable viruses and other malware, thereby possibly restoring normal boot and operation.
Avira AntiVir Premium has several improvements over the free version, including:
* Email scanning (POP3 and SMTP) 
* WebGuard to block access to malicious sites
Avira AntiVir Premium Security Suite Has some more improvements over both versions, including:
* Anti Spam for E-mail
* Firewall blocks hackers
* Backup System to protect and secure important data 
* AntiBot prevents dangerous botnets from taking over your system 
* Parental Controls blocks unsuitable websites for Children and Teens