When GRUB loads read the text at the bottom if you have a password set press p and entrer it if not I think you press e to edit the load commands. Look for the one that says kernel /boot/vmlinuz-2.4.18-5 ...
at the end of it put a space then 3
press enter and then press b too boot. this will get you into runlevel 3 so you can see what is going on.