kmistry's solution is correct, it will get you into a command prompt to be able to access the file you need to.
Once you are rescue mode, select "Command Prompt" This will put you at a bash prompt. Now you need to mount your hard drive somewhere, so:
mount -t ext3 /dev/hdX /mnt/linux (replacing X with your correct device letter/number, and the filesystem 'ext3' with your filesystem)
Now you are in the directory you need to be in, and you now need to edit the inittab file in that directory. Using your favorite command line text editor:
If not pico, change that to your choice. Now locate the line:
# Default runlevel. (Do not set to 0 or 6)
And change that 5 to a 3.
Now save the file and exit your text editor.
Reboot, and take out your boot disc. Smile and login and fix your problem. If you want to go back to starting with a GUI (after you have fixed your problem of course) simply change the 3 back to a 5.