from apple:"From page 31 of the "Welcome to Mac OS X" booklet:
If you forget your password for your computer, insert the Mac OS X CD
and restart your computer while holding down the C key on the keyboard.
When the Installer appears, choose Reset Password from the Installer
menu and follow the onscreen instructions.
Tip: Resetting an account password doesn't affect other kinds of
passwords that might be forgotten, such as your email account password,
.Mac password, secure server , Internet access password (if
you have one), Keychain passwords, or Mac OS X 10.3 Panther's master
password. "
If that does not work, follow the indications at the bottom of this page:
Macintosh OS: Lost Admin Password