A boot sector virus affects the sector of a hard disk, which is a very crucial part of a computer. the boot sector is where all information about the drive is stored, along with a programme that make it possible for the operating system to boot up. by inserting its code into the oot sector, a virus guarantees that it loads into memory during evry boot sequence.
A boot virus does not affect files, it affect the disk that contain them. perhaps this is the reason for their down fall.