A……………..sometimes called a boot sector virus, executes when a computer boots up because it resides in the boot sector of a floppy disk or the master boot record of a hard disk.

A system or boot sector virus resides in the boot sector of a disk (master boot record for hard disks, boot sector for floppies) and executes when the computer boots, before the operating system loads. Trojan horses disguise themselves as legitimate software, file viruses infect executable files, and macro viruses infect documents/files with macros.