Assuming this is YOUR machine you are trying to break into, the procedure here does not work under Tiger. I used to do the following procedure for Tiger.......It should work still under the latest update. It will basically make your machine think it's had OSX installed on it just now.
1) Boot your machine into Single user mode by holding down Command-S (apple-S).
2) Issue the following commands (without double quotes), followed by Enter:
"mount -uaw"
"cd /var/db"
"rm .applesetupdone"
3) This "hidden" file (.applesetupdone) we just deleted tells OSX that the initial setup has already been done. By deleting it, we are "forcing" OSX to do the initial setup process all over again. During this process we create a "dummy" user account. This dummy account will have admin privileges, so we can use it to RESET the password on the other account (the account you can't remember the password for).
So Reboot the machine (using "reboot" command or "shutdown -h now") and setup your dummy account, reset the "real" accounts password and optionally delete the dummy account once you get access to the "real" account.
Have Fun,
Many