First of all, if you forget your admin Ubuntu password, you will need physical access to the PC in order to recover your password.

Usually Ubuntu install in the grub menu this option:
title        Ubuntu 8.10, kernel 2.6.27-7-generic (recovery mode)uuid        393ac665-f5c2-488d-b601-b59ba1d5675bkernel        /boot/vmlinuz-2.6.27-7-generic root=UUID=393ac665-f5c2-488d-b601-b59ba1d5675b ro