Welcome to LQ!
You can use any LiveCD to boot your system. Your install CD should allow you to open another terminal that you could use to create a mount point for the partition in question. Once mounted you could then do a 'chroot';
After you get to the login then from the cli (command line);
~#mkdir /MY_temp #temporary mount point
~#mount /dev/your_device /My_temp #the device you installed to
~#chroot /My_temp #change to yours
You should be able to modify or edit whatever is needed on this filesystem.
You can 'man commands' in the above example to get a full understanding of the commands and options.