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. When the PC boots, the virus program is run and will typically install itself into memory before the operating system is loaded. A virus which infects the original boot sector on a floppy diskette. 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.