Call Apple Customer Support 1-800-767-2775, provide the Serial Number and specifications of the Mac, and for a reasonable fee, they will supply a replacement set of system discs (if available).
The discs will be for the original version of the OS that was pre-installed when the Mac was manufactured.
Get the Mac to set up an additional administrative account. You can then change the password on your old account.
Start with your computer power off. Hold down command-s. Power on your computer.
Type in the following:
The first two commands will depend on your release of Mac OS X. Look at what is typed out in the console to determine the exact format.
# Type the follow two instructions to access the startup disk in read/write. Press return after each command.
# in case of partial success repeat this command until errors go away.
/sbin/mount -uw /
#List all files. The l is a lower case L.
#The move command acts as a rename command in this format.
mv -i .applesetupdone .applesetupdone.old
# reboot your mac
shutdown -r now
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!!
Limnos adds detailed explainations:
The above the idea came from a post by JoseAranda at September 9, 2006 3:48 AM
You will need to scroll down to see this post. Search for applesetupdone
This method is mentioned in an apple support page without giving an explanation.
Once you have a new administrative account, you can change the password of your old administrative account
blue apple > System Preferences > Accounts