Understanding Viruses

In simple terms, a computer virus is a software program or a bit of scripted code designed to compromise the integrity of your PC or cause it harm. Many people wonder what would prompt someone to write a virus, but the potential reasons are almost as complex as human nature itself. Some people write viruses with financial gain in mind, looking to steal banking or credit card details from your PC. Others are out to prove their programming prowess or cause embarrassment to large corporations (such as Microsoft) by exploiting flaws in operating systems like Windows Vista. Some are teenagers, university students, or just regular folks looking for a challenge or the notoriety associated with being the person who managed to unleash the latest virus to spread worldwide. Still others want to steal e-mail addresses for the purpose of sending spam or even to use your PC as a "zombie" that can be remotely controlled to send junk e-mail messages or launch hacking attacks against different web sites and corporations. There have even been cases where someone has claimed to write a virus with the intention of undoing the damage caused by a virus that preceded it.

Ultimately, there's little point in worrying about why viruses exist-they've been around since before the dawn of the PC age, and they're not going away anytime soon. Instead, you should focus your attention on understanding the risks that viruses pose, how to remove them if your PC is infected, and most importantly how to protect your computer against virus threats in the first place.

Types of Viruses

The term virus has become a generic umbrella term used to describe almost all types of infectious and malicious code. Some people consider anything that they didn't explicitly install on their computer to be a virus, whereas others categorize virus-like threats according to their infection techniques and goals. Broadly speaking, viruses can be divided into the following subcategories:

• Viruses. Although the term is now used to generically describe a broad grouping of threats that also includes worms and Trojan horses, a virus is best described as malicious code with the capability to replicate itself. Viruses are typically attached to programs and files, infecting other programs and files as they're opened and executed. One of the key distinguishing traits of a virus is that it cannot replicate without direct user intervention of some sort, such as opening an infected file.

• Worms. Computer worms are fundamentally similar to viruses, but with one key difference-a worm can replicate itself using different network communication methods without user intervention. For example, if a worm infects one computer as a result of that system missing a critical security patch, the worm might use the infected computer's Internet connection to scan for, detect, and then infect other computers on the Internet that are unpatched and not properly secured. Because they don't require human intervention to spread, it's not unusual for a worm to infect thousands of PCs worldwide in just a few hours.

• Trojan horses. Named after the fabled wooden horse that helped Odysseus trick and defeat the Trojans at Troy, a Trojan horse is a type of virus that masquerades as a legitimate program, usually with the intent to install a backdoor program on your computer. In some cases, the Trojan horse program actually performs the legitimate task that it originally claimed to, such as functioning as an FTP client. In others, installing the Trojan horse simply installs the malicious backdoor program that allows remote users to connect to and control your PC over the Internet.

Different risks associated with each of these virus categories are explored in more detail in the following sections.

Cross Reference 

Many people consider security threats like spyware and adware to be viruses. Strictly speaking, these malware threats are different than viruses, even if their infectious aims and intentions are often similar. You'll learn more about dealing with malware threats in Chapter 10.

Virus Risks

Viruses, worms, and Trojan horses always represent a risk to PC security and your personal privacy, but some are worse than others. In much the same way that a program can be designed to do just about anything, so too can a virus. The main difference between a regular program and a virus, however, is that a virus is usually designed with malicious intent in mind. Even a virus that does nothing inherently bad represents a real risk-when unleashed onto the Internet, it's not unusual for another user to take what was a relatively harmless virus to begin with and reprogram it to carry out more damaging tasks.

Some of the more common types of damage caused by viruses, worms, and Trojan horses include:

• Using your PC as a remotely controlled zombie for the purpose of sending spam e-mail or launching hacking (denial-of-service) attacks against other computers or web sites.

• Theft of credit card information, banking details, or other personal information from your PC. This information can be used to purchase goods and services, potentially racking up thousands of dollars in charges to your accounts.

• Harvesting e-mail addresses to sell to spammers. In the world of spam, known good e-mail addresses are like money in the bank. Many viruses are designed to infect your computer and then harvest all of the e-mail addresses present in your contact lists or address books, which are subsequently forwarded to a server on the Internet. Beyond simply collecting addresses, many viruses attempt to forward themselves to users in your address book by sending those users infected e-mail attachments that appear to be coming from you personally.

• Shutting down security programs like firewalls, antivirus software, and antispyware tools. Knowing that security programs are the biggest barriers in their quest to infect or remotely control your computer, many virus writers have taken to disabling common security tools as part of their infection process.

• Installing backdoor programs that capture all of your PC activities in the background including screenshots, mouse clicks, keystrokes, and more. The information captured by these viruses can be forwarded to remote users on the Internet, and then used for a variety of purposes ranging from identity theft to blackmail.

• Infecting individual files or program templates. Some viruses are designed to infect macros or templates in programs like Microsoft Word, causing all documents you create to be infected as well. When an infected document is sent to another user and opened on her PC, the virus also infects that computer.

• Deleting files, formatting hard drives, or altering critical system settings. Some viruses are simply designed with destruction in mind, be it deleting your personal files, formatting or erasing disks, or even changing system configuration settings to the point that your PC will no longer start and your operating system will need to be completely reinstalled.

Ultimately, all viruses are dangerous and must be taken seriously. Although some represent a bigger risk to your security and privacy than others, it's critical to assume that every virus is a danger to either you or your computer, and take appropriate action immediately.