Two choices:
1) You should be able to do cntrl-alt-F6 to get a prompt in run level 4.
2) You can boot the 1st install cd. At the prompt log in as root and then mount your / partition
Code:
mkdir /mnt/root
mount /dev/hda1 /mnt/root
then edit /mnt/root/etc/inittab to go into run level 3. and reboot.