A computer virus is a program that infects part of a system, causes some sort of damage or data loss, and produces copies of itself to spread to other computers. The damage viruses cause can be minor or catastrophic, depending on the whims and abilities of their authors. Some viruses have been known to destroy the data structure of an entire hard drive or even to damage the infected computer's BIOS chip (without which a computer doesn't work at all); others simply target and destroy one type of file, such as program executables or documents.

Worms behave almost exactly the same as viruses, except that they don't damage data--they exist only to reproduce themselves and spread to other computers. It's possible to weather a worm infection without any disruption to the normal, day-to-day use of your computer. However, if a worm infects a company's network, it can set off a deluge of interoffice e-mail, which can bring down the company's computers.

Trojans are often used in conjunction with denial-of-service (DOS) or distributed denial-of-service (DDOS) attacks, which are discussed below. In most cases, you can easily avoid a trojan infestation in the same way that you can thwart viruses or worms: by not opening suspicious e-mail attachments from both unknown and familiar parties. Trojans are often found in downloadable software from questionable sources, such as hacker sites and warez (illegally reproduced commercial software) servers.

More sophisticated than worms, viruses, and trojans are denial-of-service (DOS) attacks. These aren't programs at all, but actions undertaken by nefarious parties that result in the inability of users to perform common Internet-related functions, like downloading their e-mail or accessing a Web site. The most common form of DOS attack involves hackers triggering dozens, hundreds, or even thousands of bits of false data packets toward a particular Web site. The attack is intended to cause so much false traffic that legitimate users are unable to access the site.

Unbeknown to you, your computer may even assist such attacks. A particularly nasty form of DOS assault, called a distributed denial-of-service (DDOS) attack, involves hackers controlling unsuspecting PCs all over the Net through the use of trojans. A hacker or group of hackers will trigger trojan-controlled computers to simultaneously send massive amounts of false data to the same target. The result is usually tons of data overloading a Web address, choking off real traffic and sometimes crashing the targeted Web server. The best way to prevent this from happening is by installing a firewall, or a program designed to block unauthorised access to and from your computer or network. See more on firewalls in the last section of this guide.

Your first line of defence against unseemly Internet activity should be to adjust your own habits:

• Don't open e-mail attachments from people you don't know--even viruses like the famous Love Bug aren't dangerous until a user activates them, usually by opening an e-mail attachment.
• If you receive an e-mail with an attachment from someone you do know, check the file's extension before proceeding. Image files (with extensions like .JPG, .GIF, .BMP, and such) are usually safe to open, as are text files (which have the extension .TXT). Files that may be dangerous are program executables (.EXE, .BAT, .COM, .PIF), Microsoft Word documents (.DOC), and Visual Basic scripts (.VBS). Never open a .VBS file. Delete the e-mail immediately.
• Regardless of the attachment's extension, save it to a folder on your hard drive and scan it with an antivirus program before you open it.
• When you download files and programs, scan them with your antivirus program before executing them.

If a virus does crawl into your computer, your antivirus software will inform you of your options. Often, an antivirus program can remove viruses from files or boot areas without the need for any further recovery. You may have to boot your system with the emergency diskettes the program made for you.

Sometimes, your antivirus program won't be able to purge an infected file of its virus. The file should be deleted and replaced with a clean copy. Your antivirus software will help you through the process.

Firewalls block unauthorised access to and from your computer or network. Information is transferred to and from your PC via open ports, which are basically data conduits. Firewalls block vulnerable ports that can be used by hackers to control your system with trojans or perform other malicious tasks. As such, firewalls are especially useful in preventing your computer from taking part in a DDOS attack, or from being accessed by hackers even if you've already unknowingly installed a trojan.