Viruses are a threat to every computer user - whether at home, on the road or in the office. As computer users turn to the Internet for social networking, work, travel, hobbies and a variety of informational needs, the risk of encountering a virus continues to increase. Computer viruses are self-replicating programs that access your system via an e-mail attachment, a download or a Web site. Developed by pranksters at first, viruses are now used by cyber thieves and hackers with malicious motives. Boot and program viruses were the first two varieties to create mayhem. They remained the most prevalent until the late '90s, when script and macro viruses appeared.

Forewarned is forearmed. Here's a quick run-through of the major types of viruses to help you understand how you might inadvertently transmit them.

• Program viruses can travel across the Internet as e-mail attachments or be spread via media, such as a CD. This variety will attach itself to executable files. Every time you run the program that governs the infected file, the virus is able to duplicate and attach itself to other programs. Sharing software programs (via disk or network) or downloading programs from the Internet is how program viruses are spread. The first thing a program virus does is insert commands and settings into the host computer's operating system. If the virus remains undetected by an antiviral program, it will achieve full access to log records, and the user will never see a warning message.
• Trojan horse viruses are a type of program virus; however, they don't replicate themselves once they access the host. They typically come as an email with some sort of strange subject line or an offer contained in an attachment or embedded link. Regard anything like this as a major red flag. Don't investigate strange or unexpected offers. If you open the email attachment, you might unleash a variety of problems that could attack your hard drive or provide a launching pad for a boot virus or a worm.
• Boot viruses hide in the boot code for media devices (floppy disks, CDs, DVDs, etc.). When you boot from the infected device, the virus attaches itself to other boot files on your hard drive. Since the decline in usage of floppy disks and other transferable bootable media, these viruses are not as prevalent as they once were.
• Macro viruses invade a computer via the macro commands for programs and attach themselves to specific files. They access computer hosts just like a self-executing virus via email or file downloads from the Internet. Macro viruses can be the hardest to detect. Many popular applications such as Word and Excel disable the execution of other macros by default. This helps protect users from accidentally launching a virus and has lessened the proliferation of macro viruses. Sometimes users might need to adapt their software security settings. With a few changes, these programs can be configured to accept trusted macros, if need be. Computer users can disable macros safely if they are only dealing with known sources, such as the company's technology department. When downloading programs from the Internet, be sure you know the source of the download and can verify that it is virus free.

Running antivirus software regularly is a must, but it is equally important to understand the dangers that viruses pose. Common sense and caution are the keys. Remember that something sent to you in good faith by a friend might have been infected with a virus on the server where it was previously stored. Erring on the side of caution could save you money and a whole lot of aggravation.