graphical login is chosen by X. So a runlevel that starts X will have a graphical login of sorts (psyche).
You need to get your hands on knoppix/morphix/slackware livecd to use as a recue disc. Once you get to a shell with the rescue disk:
mount /dev/root /mnt/mylinux
mount -t proc proc /mnt/mylinux/proc
mount /dev/anyotherpartitionsyoumayhave /mnt/mylinux/whatever
then do something like:
nano -w /etc/rc.conf
Check the run leve setting, and make sure it isn't 0 or 6. Making it 5 is a good bet.
This is of course assuming it's runlevel... It may even be that something or other has added reboot to a perfectly innocent runlevel making the system halt/reboot.