A virus is a computer program that replicates itself by modifying system or application software and then traveling from computer to computer, via disks or a network. Most viruses are harmless, but some viruses can erase files on your hard drive, slow down your computer, or do other kinds of damage. You can protect your computer against most types of viruses by installing virus protection software. See our security page for more information.

Trojan Horses are impostors — files that claim to be something desirable — but, in fact, are malicious. A very important distinction between Trojan horse programs and true viruses is that they do not replicate themselves. Trojans contain malicious code that when triggered cause loss, or even theft, of data. For a Trojan horse to spread, you must invite these programs onto your computers — for example, by opening an email attachment or downloading and running a file from the Internet. See our security page for more information.

Worms are programs that replicate themselves from system to system without the use of a host file. This is in contrast to viruses, which requires the spreading of an infected host file. Although worms generally exist inside of other files, often Word or Excel documents, there is a difference between how worms and viruses use the host file. Usually the worm will release a document that already has the "worm" macro inside the document. The entire document will travel from computer to computer, so the entire document should be considered the worm. See our security page for more information.

Ad-ware programs gather information from your computer for marketing purposes. The information is used to decide, for instance, which advertisements to display when you are on a specific Web site or which advertisements to display in pop-up windows on your desktop.

Spyware programs also gather information from your computer, but are less likely to have obtained your authorization to gather that information. Spyware may be more difficult to identify and remove, and may use that information for other purposes in addition to marketing. In the remainder of this document, the term malware (short for malicious software) is used to refer to both ad-ware, spyware, joke programs, and dialers. See our security page for more information.