Same issue when installing Ubuntu on my machine. I tried reinstalling GRUB, several times, and nothing- still wouldn't work. I do know that it is with the GRUB boot loader, just not sure how to fix it. Before I usually do anything major, I create a Ghost image of my HD, verify the integrity, and then proceed. Needless to say, I've learned my lesson from things like this
You can try going through the setup process again, leave the file systems intact, and try to install the GRUB boot loader again.
Whatever happened to Lilo? I had a better success rate out of it