In some cases failure to do so will prevent antivirus programs from detecting and removing certain viruses from the computer. Those areas on diskettes or hard disks that contain some of the first instructions executed by a PC when it is booting. Boot records must be loaded and executed in order to load the operating system. Viruses that infect boot records change them to include a copy of themselves. These viruses are particularly serious because information in the boot sector is loaded into memory first, before virus protection code can be executed.
Some viruses always attack the first physical sector of the disk, regardless of the disk type.
COM is short for command, being a file that contains instructions that can do something on your computer. COM files are for DOS based systems and tend to run faster than EXE type programs. Viruses will often infect COM files. When the COM file is run the virus is run as well, often loading it into memory. A viral program that does not actually attach to another program, but which uses a similar name and the rules of program precedence to associate itself with the regular program.