Viruses: different types and how they work
A software virus is a program designed to change the way your computer operates without your consent. Some viruses are intentionally written to delete files, corrupt programs or erase disks. By definition a virus replicates, spreading itself by attaching copies to other files.
Macro viruses spread whenever you open an infected file and can be transferred across platforms. They can destroy and change data saved on your computer.
Trojan horse is a term used to describe a computer program that appears to serve a useful purpose or have entertainment value, but in fact is designed to damage files or install another virus on your computer. A Trojan horse is technically not a virus itself because it does not replicate. It is a means of delivering a virus. However, any file that contains a Trojan horse cannot be repaired and must be deleted immediately.
Worms are programs that copy themselves without necessarily altering other programs. They damage systems by making so many copies of themselves that they can slow down a computer. Like Trojan horses, worms cannot be repaired, so any file that is infected must be deleted completely.
You should be suspicious of any program that you download from a newsgroup, Web site or e-mail attachment. Using an up-to-date virus protection program, you should scan everything you download.