Hi and welcome,
You will need the install disk then have a look here
http://support.apple.com/kb/ht1274
Hope it helps.
EDIT:
Should have added that you cannot reset the password with OS discs older or newer than the version currently on the system.
And basically this is what you
Boot Mac with the Install Disk in the drive, wait until the Apple Logo Appears, and when it boots, select your Language and click the arrow.
From the Installer Menu (Menu Bar) select Utilities>Disk Utilities and Reset Password. Now follow the prompts.
But first read the link I gave.