Antivirus software provides an essential layer of protection from a multitude of virus, trojan, worm, spyware, adware, dialer, keylogger and rootkit infections. Traditionally antivirus programs just detected viruses and spyware removers just detected spyware but nowadays the boundary between antivirus and spyware removers is unclear. Most current antiviruses have at least reasonably good detection rates of all forms of malware.
Malware includes viruses, trojans, worms, spyware, adware, dialers, keyloggers and rootkit threats that perform malicious activities on a computer. With the huge increase in malware, antivirus software cannot keep up with detecting all of it. But using more than one real-time antivirus uses much more system resources, can cause conflicts and even reduce protection. So I recommend you only choose one antivirus for real-time protection. Instead, you can increase your protection using other security software.
Avast! Free Antivirus is an excellent free antivirus that many users prefer. Its detection rates is on par with AntiVir. Avast has the most features, with full real-time capabilities, including web, e-mail, IM, P2P and network shields, boot-time scanning, and a behavioural blocker. Avast is light on resources.
These free antivirus programs are excellent software that provide a real alternative to the major commerical antivirus products.
You can increase your protection if you run on-demand scans with another antivirus. On-demand scans can be run regularly to check for viruses, trojans, worms, spyware, adware, keyloggers, rootkits and other malware that may have been missed by your main antivirus scanner. If you have a good preventive security strategy in place, however, the extra protection an on-demand antivirus scanner offers is minimal.
You can also use one of the free antiviruses mentioned above as an on-demand antivirus scanner. One advantage is that you can set scheduled scans if you wish.