It is never a good idea to run two antivirus scanners simultaneously. However, if only one of the scanners has real-time protection enabled and the second scanner is used only to manually scan selected files, they may possible co-exist peacefully. In some cases, an antivirus scanner will not install if it detects another antivirus scanner already installed on the system.

Despite the name, antiviruses have traditionally taken on the task of handling not just viruses but also worms and to a lesser extent trojans. The name antivirus merely reflects the common layperson use of the term virus as a catchall for all types of malicious software. On the other hand, some people recommend the use of specialized anti-trojans to supplement antiviruses because they fear that antiviruses are more focused towards handling worms that infect millions and are slower on handling trojans which are not self replicating. The number of quality anti-trojans are however in decline and many have began to focus on spyware and ad-ware instead. (TDS-3 was discontinued, Ewido has being renamed to AVG anti-spyware).

Antivirus fail for many reasons. Chief among them is that they do not have a signature for a new malware encountered and is not caught by the heuristic either. Even if they do have a signature existing malware can be modified to evade antiviruses using various methods -hex editing, using packers (packed malware) unsupported by the antivirus (also see server-side polymorphism), polymorphic (encryption) , metamorphic (replacing code structures with equalavant code) or combination of methods. As such particular antivirus might have a signature for a given malware, but fail to detect it if it is packed. Lastly the antivirus might have a bug in its detection routine under certain situations.

Antivirus technology is essentially based on blacklisting technology, where known "baddies" are listed. The problems with this are pretty obvious : one generally has to list every known instance of malware and there are literally thousands created every day (heuristics help but generate too many False positives), and in view of the rapidly increasing numbers of malware, recently interest as shifted towards a strategy/technology known as whitelisting. Whitelisting turns blacklisting on its head, instead of listing all the bad guys, it lists all the good guys in a "whitelist", and denies all others.

These terms (as opposed to on demand scanning) refers to the ability of the antivirus to scan files on the fly as they are either created/read (opened)/modified or executed (or some combination of) without any human intervention. Real time protection of different antiviruses varies, some will scan files the moment they hit your hard-disk while others will scan them only before you execute them and many allow you to configure it as you like. You can see this difference when you are surfing the net, and some harmful file is deposited in your browser cache. Some antiviruses will scan this file once it is created and instantly alert, while others will stay inert unless it is executed. Some people prefer the former, but arguably, the later gives you equal amount of protection since a file that isn't run is harmless. Also the later approach is likely to be less computationally taxing on the computer.

CyberScrub AntiVirus Lifetime Edition - Secure your computer from attack Destroy viruses, worms and Trojans. Includes 5 Year License. Includes KERIO FIREWALL. Checks the computer and diskettes for viruses on demand. Continuously checks for viruses in all files upon their opening or executing. Scans for viruses in MS Outlook Express email databases. Removes viruses from ZIP archives. Streamlined updating of the anti-virus database.

PC Tools AntiVirus is a full anti-virus package designed to protect your computer from virus, worm and Trojan horse threats and infections by: scanning your computer system for these types of infections and quarantining or deleting them, and providing your computer system with real-time background protection, preventing threats from infecting your computer and propagating while you work.

Desktop Armor watches your computer for changes, notifying you when something is altered to confirm you want to keep or reject the change. This catches many of the changes made by a virus, trojan or worm; protecting your computer from harm. It helps you protect your computer from hackers and worms by disabling rarely used Windows services, it helps you protect your privacy by blocking popup ads, "Phishing" scams, and more.

Spy Emergency is anti-spyware software that fast and secure removes spyware and other internet infections from your computer. With its intuitive user interface, spyware removal is very simple for every user. Spy Emergency Features:Scan of your system memory, Scan of your registry, Scan of your storage, System directory scan, Tracking cookies scan, Host file scan, VX2 complaint hijackers scan, Automatic LSP stack repair.

Adware Remover easily detects and removes all forms of spyware and adware with a few clicks. It also detects dialers, Trojans, malware, data-mining, aggressive advertising, parasites, scumware, browser hijackers, and tracking components.