In order to get into a system at boot time to pass params to the kernel or boot
a system that won't boot do the following:

When GRUB begins to load (see /boot/grub/grub.conf) it has a timeout
for a few seconds. During this timeout you can hit c which gets you to 
the GRUB command prompt. (Your mileage may differ if you are using LILO)

At the GRUB prompt enter the following:

###################################
root (hd0,0)       #you can see if you are booting hd0,0 in your grub.conf file
kernel /vmlinuz-2.6[tab_completion] root=LABEL=/1 ro rhgb init=/bin/sh
initrd /initrd[tab_completion]
boot
mount -orw,remount /
####################################
You now have a root shell without haviing entered a root password