You need to boot from the install discs that came with your MacBook and select to change the user's password from the menu. Once you've inserted the disc into the MacBook, reboot and hold down the C key after you hear the startup chime until you know that it's booting from the disc.
• Apple iMac G5 17" (2 GHz G5) - Mac OS X 10.5.8/Ubuntu 10.04
• Asus Eee PC 901 (1.6 GHz Atom N270) - Fedora 13
• Apple Macintosh Quadra 650 (33 MHz MC68040) - Mac OS 8.1
• "JHVH-1" (2 GHz AMD Athlon XP 2400+) - Slackware 13.1