If you boot into single user mode you can reset your password.
This can by hitting e on your grub menu with your desired kernel, selecting the kernel line on the next screen and hitting e again, adding "single" (without the quotes) to the end of the line, hitting enter, and then typing a b to boot the thing.
You will get dropped at a shell prompt where you can set your password using passwd. Then exit the shell and regular bootup should continue.
This is why I always use a grub password... and don't allow just anyone physical access to my machine
For more info on how to get around in grub (and just in case those keystrokes I gave you weren't 100% correct... did it from memory as I don't currently have a linux box I can reboot) check out the Grub Manual