If you know the location for key components, etc. you can boot to a given device/partition/etc from a grub command line.
When you are at the grub boot menu, type 'c' to get to the boot prompt.
Next, you enter a line similar to (borrowing from my current configuration):
kernel (hd1,0)/boot/vmlinuz root=/dev/hdb1 vga=0x31a splash=silent resume=/dev/hdb2 showopts
Once you change the hd and partition parameters to match your configuration in the above line, hit enter.
Now you need to enter one more line:
Question though: is there any reason you couldn't just boot to single user mode? Try entering either "init 3" or "linux 3" at the bootprompt and it "should" boot to runlevel 3, which will enable you to reconfigure the box (assuming you don't have xdm / rcxdm starting in runlevel 3), and if that fails, you could try single user mode, which is init1, and disable xdm / rcxdm by editing the init.d scripts (change id:5:initdefault: to id:5:initdefault in /etc/inittab )