A computer virus is a program specially made to auto replicate and spread. It infects many computers, without a pre-agreement with the computer’s owner.
They spread by attaching to some programs like: .exe file, .com files, and recently to WORD, Excel, or even to HLP files. Some of the viruses can affect the boot sector of the HDD.
When an infected file is executed, or when the computer is started using an infected CD or diskette, the virus starts its work. Sometimes, the virus remains in the computer memory just to infect the next program to be run.
The dangerous viruses have the ability to execute some actions. Sometimes those actions are annoying (like the display of some response on a certain date and time, or as a feedback to an user action), or they can be very annoying – like slowing the computer performance. There are viruses that can cause a very big trouble, destroying files of data, documents or making the computer unusable.