Before you can understand what a boot-sector virus does, it's important to know what a boot sector is. A hard drive is comprised of many segments and clusters of segments, which may be separated by something called a partition. There has to be a way to find all the data spread across these segments, and so the boot sector operates as a virtual Dewey Decimal system. Each disk also has a Master Boot Record (MBR) that locates and runs the first of any necessary operating system files needed to facilitate operation of the disk.
When a disk is read, it first seeks the MBR, which then passes control to the boot sector, which in turn provides pertinent information about what is located on the disk and where it is located. The boot sector also maintains the information that identifies the type and version of the operating system the disk was formatted with.
Obviously, a boot sector or MBR virus that invades this space on the disk puts the entire operation of that disk at risk.
A boot sector virus is spread via infected floppy disks. This typically 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 happens to be 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".
This is a standard error message and is not in and of itself indicative of a boot sector infection. All it means is that a non-bootable disk is contained in the drive the computer is first trying to boot from.
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. Of course, if the disk was bootable, they would not receive the error noted above, but will simply be booted to a DOS screen.