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 properly. Wikipedia has a good description of how individual disks are partitioned. It's a good place to start if you don't have a basic understanding of things like the MBR, primary and extended partitions and the different partition types.
Every PC goes through a process when you turn the power on called the boot process. One artifact of the original PC design is something called the Basic Input Output System or BIOS. This is typically kept in a Read Only Memory (ROM) or Flash ROM that can't be altered without running a special program to change it. When the computer wakes up it starts trying to execute instructions at a fixed location -- the first address of the BIOS. After the BIOS does its thing it hands over the process to the boot loader.
That's where GRUB comes in. The name GRUB comes from a project named the Grand Unified Boot loader. It's the primary boot loader used in all the mainstream Linux distributions. In most cases GRUB is loaded into the MBR of a disk drive and is passed control from the BIOS. GRUB can also run from a floppy disk or other removable media.
GRUB is unique in that it comes with a menu of options and even a command prompt if you need to interact with it. The options menu is driven by a structured text file that you can edit and change to meet your needs. GNU GRUB is the name of the open source project with the goal of improving the overall boot process to make it faster and more secure. Check out their web site and wiki for more information on GRUB itself.