A boot sector virus infects the boot sector of a drive and is spread via infected floppy disks. This usually occurs when users inadvertently leave a floppy disk in drive A:\. When the system is next started, the PC will attempt to boot from the floppy. If the disk is infected with a boot sector virus, that virus will infect the boot sector of the user's local drive (C:\).
Unless the floppy disk is a bootable system disk, the user will simply see a standard warning that the drive contains a "non-system disk or disk error" and the user will be prompted to "replace the disk and press any key when ready". Most users will realize a floppy has been left in the drive, remove it, and reboot the system, unaware they may have just infected their system with a boot sector virus.
Compared to other types of malware, a boot sector virus can be fairly benign - simply taking room up in memory. However, a boot sector virus can also contain a malicious payload.The simplest method to prevent a boot sector viruses is to change the CMOS settings to boot from the local C:\ drive first, rather than from floppy. Most modern BIOS is already configured to boot from the hard drive first.