This is the reason I like terminal to install grub.
but you need to be more specific which rescue cd you are using. And you have not posted if grub is no longer in your mbr so it may be ok just your menu.lst or grub.conf that needs changing.
in which case hard drive boot to grub and follow my tips on trouble shooting to use commands to boot to the old kernel
2) knoppix should be easier if you can get your hands on one to see what files are where if the trouble shooter is too hard for you or post results