The most simple way to reset the user password is to pop in the Install CD/DVD and go to the install menu. There is an option to reset the passwords.

Another option is to remotely mount the drive useing firewire target disk mode. ("T" at boot)

You should learn NetInfo Manager, it does everything that /etc does.

finally use ssh you can remotely log into the machine and reset any password.

ssh root@yourhostiphere
the root password

cd /Users
select a user name
su username
passwd
and change the password.
That should do the trick