Cause: You’ll face this problem when a boot sector virus corrupts the MBR (Master Boot Record). Boot sector corruption can generally be attributed to viruses, but sometimes, tweaking the boot loader can also leave the system with a corrupt boot record.
Solution: For Windows 9x/ME and DOS, one can use the FDISK command to repair the boot sector. Boot the system from a bootable floppy or CD-ROM and locate the FDISK command. Once located, type in “FDISK /MBR” and hit [Enter]. Wait for the process to complete, and your boot sector should be fixed.
For Windows NT/2000/XP, boot via the bootable CD-ROM and enter the repair console. Here, type in “FIXMBR” and hit [Enter] to restore the boot sector.