A computer virus is a term applied to any program designed to damage data stored on a computer system or network. The affected data may include documents, photographs, applications or worst of all system files that are vital to the proper operation of the computer system. The virus is written by an ill-intentioned computer programmer and spread to other systems by means of e-mail, floppy disks or downloaded software.
A computer virus only affects the software of your computer, not the hardware. In a worst case scenario where a virus has taken control of your system and caused irreparable damage, you may still regain your system by formatting (deleting) the hard drive and reinstalling your OS (Operating System, such as Windows 95, Windows 98 etc.). This is a painful solution of course as you lose all of your data! There are better solutions to prevent this from happening, as I will discuss here. Firstly here is a more detailed description of the main types of viruses:
"Trojan Horse" is a very apt name for a particularly nasty type of virus. Although technically not a virus, a Trojan Horse is in many ways a more important treat to be aware off. A Trojan Horse must be installed on your computer to become effective. This may be achieved by someone having physical access to your computer, e.g. in an office, or in the more common way by you downloading and installing a seemingly harmless pieces of software. Like the soldiers hidden within the fabled wooden horse, the Trojan Horse has the ability to attach itself to another program then install itself onto your machine covertly when you install the program it resides within.
Many Trojan Horses are key-logger programs that record every key press on your keyboard and store them in a log file. This log file is then e-mailed to the person who sent the Trojan Horse to you in a convert way every time you access the Internet. Everything you type; e-mails, letters, credit card numbers, passwords, phone numbers etc. are all logged and received by the hacker, your privacy is completely compromised. Some more advanced key-loggers or password sniffers also keep account of all of the web sites that you have visited and when.
Worms have the ability to replicate themselves, and generally do not require human interactivity to do so. They may be designed to e-mail themselves to other computers. Worms copy themselves from computer to computer rather than file to file like most viruses, and because they require no human help to do so, they can spread much more rapidly through a system than a normal virus.
A Worm may arrive in an unsolicited e-mail in an attachment that tempts you to open it, once you do the worm spreads to your computer. E-mail Worms that have the ability to replicate and e-mail themselves to many people simultaneously have resulted in the crashing of e-mail severs in the past due to the excessive strain of all the traffic!
Zombies may remain dormant on your computer system for an extended period of time. They are normally programs used by hackers in a coordinated attack on a system, whereby the hacker triggers the Zombie to aid his/her attack on a system or network. Zombies by themselves normally cause no damage, but their presence on your computer is of course unwelcome. Like many other kinds of viruses, they are normally received by e-mail.
The number one rule is to always be aware of unsolicited e-mails, especially ones of an "unsavoury" nature such as pornography-related e-mails. Any executable .EXE files should not be opened unless you trust the source, e.g. you got it from your mother! Otherwise, be very vigilant.
Another source of viruses is downloaded programs that promise to entertain or be useful. Always be wary of what software you install, and check the company's credentials. Downloaded "Warez" (illegal pirated software) is another source, so be very careful of this.
Backup all important data and applications onto floppies, zip disks or CD ROMs. If the worst does happen, at least all of your hard work will not be lost with the deletion of your hard drive.
There is only of type of computer user that does not require anti-virus software to be installed: those that have NO Internet connection, do not share floppy disks or zip disks with anyone, and if they live in a remote log cabin that might help also. For the rest of us, you really should go out straight away and buy some decent software. They are many brands available at good prices, so I will avoid mentioning particular brand names. All provide similar functions at various levels of success.
Anti-virus software resides in the active memory of your computer, and takes control of it to alert you to an active virus present on your machine. If the software cannot repair the infected file, it will quarantine this file or give you the option of safely deleting the file.
Anti-virus software may also be used to scan your hard drive, floppy disks, zip disks or CD ROMs. It may also be used to scan attachment files to e-mails. The important thing to remember is that new viruses are being discovered daily, so if you have anti-virus software installed then make sure that you keep it's library of known viruses up-to-date, otherwise you will have no protection against the latest batch.
Firewall technology is nothing new; it has been present on most Internet and LAN servers for many years. What is new is that firewall technology is now available on a smaller scale for the single user with one computer connected to the Internet. While not as immediately important as anti-virus software, if you are serious about your security and protecting your privacy online, you might consider buying a firewall.
Firewall software acts as a secure barrier between your computer and the outside world. It monitors all traffic to and from your computer, and decides whether or not this is normal Internet activity or an unauthorised security risk. To the hacker, firewall gives the impression of your computer not being there, or at very least being difficult to locate. Furthermore firewall provides additional protection against Trojan Horses as it will block the unauthorised e-mailing of the key-log file to it's intended recipient, and alert you of the Trojan Horse's attempt to do so.
Like anti-virus software, there are many brands of firewall software on the market. Many companies now offer anti-virus and firewall technologies bundled together at a reduced price, which generally prove to be excellent value for piece of mind.
The likelihood of being the victim of a virus increases with the level of your web presence. If you browse the Net only occasionally, never download software and do not open attachments, you run a very small chance of being affected. If you have a web site (or several!), many e-mail accounts and receive many unsolicited e-mails per week, you need to be more careful about your web security than the average person.
Attack from viruses is not the only thing you need to worry about on the Net, you may also fall victim to hackers or fraudsters. Internet crime is now a genuine reality and no individual or business on the Net can be complacent about this. If you follow the above advice on viruses you should have no virus-related problems, I intend to write on other Internet related personal security issues at a latter date.
Updated 2020 : note that the above post is out-of-date, given this post was originally published in 2001, but is left here for archival purposes.