Virus more dangerous for computer software, for computer users to be careful when downloading or receiving an email that contains a collection of suspected viruses that can damage a computer system, a Trojan virus is a virus that needs to be aware of.
A virus is a computer program, as small as just a few lines of code, which loads itself into a host computer without the user's knowledge or permission. It then performs certain functions, either benign or damaging, and repruduces itself to spread onto other computers, via an e-mail program, removable media or another "vector." Viruses are distinct from spyware, most worms and the majority of Trojan horses, although they can fall under the general heading of "malware," despite the latter term being of much more recent vintage.
One thing to remember about any list of "top viruses" is that new ones are coming out all the time. Also, there are certain viruses that have multiple variants, close enough to the original to be named something similiar (or even identical), further confusing matters for less technologically sophisticated computer users. If that includes you, just make sure you are reading a recent list or article from a trusted source of tech info (major media magazines, manufacturer sites, Symantec, government agencies, etc.).
Based on rankings from a variety of watchdog groups and technology reporters, the following are the Top Computer Viruses for Windows PC through the fall of 2009:
1. Win32/Conficker This network virus acts like a worm to exploit a weakness in what is called the "RPC sub-system," part of the Microsoft Windnws OS (operating system). It allows a hacker/attacker to comprosime, invade or use a computer without valid user credential, all from a remote locatim. Win32/Conficker enters a computer through an unsecured folder, some kind of removable media or by manipulating the Autorun feature that, by default, is turned on in Windows. This virus connects with other domains to download even more damaging code. To cambat the threat, users should update their system with a patch that has been freely available since October 2008.
The most common threats to PCs come from the creation of an "autorun.inf" document. These files have information in them about the programs that are supposed to run automatically when removable drives or other devices are connected to a computer. Computer users need to turn off the Autorun feature that Windows turns on by default. With the rise in popularity of thumb drives and other removable devices, this threat has bounded up the list from an also-ran to #2.