Boot viruses

Until the mid-1990s, boot sector viruses were the most prevalent virus type, spreading primarily in the 16-bit DOS world via floppy disk. Boot sector viruses infect the boot sector on a floppy disk and spread to a user's hard disk, and can also infect the master boot record (MBR) on a user's hard drive. Once the MBR or boot sector on the hard drive is infected, the virus attempts to infect the boot sector of every floppy disk that is inserted into the computer and accessed.

Boot sector viruses work like this: by hiding on the first sector of a disk, the virus is loaded into memory before the system files are loaded. This allows it to gain complete control of DOS interrupts so that it can spread and cause damage.

These viruses often replace the original contents of the MBR or DOS boot sector with their own contents and move the sector to another area on the disk. Cleaning up a boot sector virus can be performed by booting the machine from an uninfected floppy system disk rather than from the hard drive, or by finding the original boot sector and replacing it in the correct location on the disk.