Viruses perform without the permission of the user and without warning the user of the consequences of their actions. Also, viruses do not print error messages when they encounter an error. They simply recover or attempt to recover without asking users to assist in correcting the error. In other words, computer viruses are designed to operate secretly. This way they can accomplish their missions and not be compromised by user input.
Computer viruses can do anything a legitimate software program can, and do it in secret.
They specialize in altering, overtaking, and infecting files stored in the disk boot sector. The disk boot sector is the area of the system boot disk where start-up command programs are stored. The infection of disk boot sectors provides numerous advantages over all other means of infection for rogue programmers. First of all, boot sector infectors are loaded immediately upon system start-up, when operating systems files are normally loaded. Since the boot sector infectors are the first programs executed during any computing session, they are able to remain resident and active at all times.