"If you've ever tried to set up a dual boot system, more than
likely you have also managed to mangle the master boot record (MBR)
of your main system drive at least one time. Once corrupted you
typically have a couple of options. One of the most obvious ways is
to boot from a CD-ROM distribution and reinstall the OS. It's
probably not the quickest fix but it usually does work. You could
accomplish basically the same thing with a bootable USB disk if you
happen to have one.
"Super Grub Disk (SGB) is a handy alternative that works in a
few seconds. You'll find versions you can burn to a CD-ROM, USB
disk or floppy disk. Booting from one of these media devices
presents you with a menu of options that should help you get your
system configured properly. It's also a tool capable of leaving
your system unable to boot if you set the options wrong.
"The key to using it correctly is to understand how a disk drive
is organized and what you need to do to get it to boot