1. Insert the Mac OS X 10.5 Install disc, then restart the computer while holding the C key.
2. When your computer finishes starting up from the disc, choose Utilities from the Installer menu at the top of the screen. (In Mac OS X 10.4 or later, you must select your language first.)
Important: Do not click Continue in the first screen of the Installer. If you do, you must restart from the disc again to access the Menubar at top.
1. Insert the Mac OS X Install disc, then restart the computer while holding the C key.
2. When your computer finishes starting up from the disc, choose Reset Password from the Installer menu at the top of the screen. (In Mac OS X 10.4 or later, you must select your language first.)
Important: Do not click Continue in the first screen of the Installer. If you do, you must restart from the disc again to access Reset Password.
If not OSX Server edition...
This was Posted by: JoseAranda at September 9, 2006 3:48 AM
"OK, restart your computer, hold down Command-s and type in the following:
/sbin/fsck -fy Enter
/sbin/mount -uaw Enter
rm /var/db/.applesetupdone Enter
# The rm command is the remove command which deletes the file.
# Robert: I'd rename the file via: mv /var/db/.applesetupdone /var/db/.applesetupdone.old
reboot Enter
Once you've done that the computer reboots and it's like the first time you used the machine. Your old accounts are all safe. From there you just change all other account passwords in the account preferences!!
end of posted by: JoseAranda"
Reset OS X Password Without an OS X CD...
http://theappleblog.com/2008/06/22/reset-os-x-password-without-an-os-x-cd/
Admin Hack...
http://www.hackmac.org/?q=node/4
Starts up like the first time you buy a new Mac, but after filling in all that info again, you should have access to the computer and the other Users & files will still be there... give the new User a different name than an existing one.