5 Top Computer Viruses

By Author: Vivek Shaker
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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 reproduces 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 similar (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 5 Computer Viruses for Windows PCs 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 Windows OS (operating system). It allows a hacker/attacker to compromise, invade or use a computer without valid user credentials, all from a remote location. 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 combat the threat, users should update their systems with a patch that has been freely available since October 2008.

2. INF/Autorun
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.

3. Win32/PSW.OnLineGames
There is a family of virus-like Trojans that were created to wreak havoc on game players by stealing user credentials and making off with other personal or financial data. The purloined information is sent to the instigator's PC, where he or she can cause some serious damage with it. Gamers should remain on high alert as this threat is still being discovered in large numbers across the globe.

4. Wind32/Agent
Yet another serious threat to computer users is the Wind32/Agent. It reproduces itself in temporary locations, steals data from the compromised PC and adds spurious entries to the all-important Windows registry. By creating a number of files at various spots in the PC's system folder, it is able to run on every single startup. This is how it assembles a complete log of information from the infected PC before transferring it to the instigator's computer over the Internet. A good defense is a strong anti-malware program, but users should also disable the Autorun facility and resist opening any unknown files.

5. Win32/FlyStudio
This modifies settings of the PC victim's Internet browser to affect answers to search queries and thereby direct the user to specific advertisements. This particular threat has primarily targeted computer users in China, although it has been reported North America, too. In an interesting turn of events this malicious code is distributed by a completely different malware family.

Although these viruses are all targeting Windows PCs, the Macintosh family is not entirely immune. It is still true that there are fewer malware and virus attacks on Macs, and it is not simply because there are fewer of them. Fact is, there are millions of Macs in the world, but the Mac community is more cohesive and the underlying OS is more robust for resisting incursions.

In mid-2006, Sophos Anti Virus identified the first "real virus" for Macintosh OS X, which was named "OSX/Leap-A" or "OSX/Oomph-A." In the form of a worm or a Trojan horse the threat is spread by instant messenger (IM) software, and sends itself as a file named "latestpics.tgz" to contacts on the infected user's "buddy" list. It does no damage to the OS besides invading it to replicate itself to other users.
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