If you are using 10.7 or later version of OS X, then you can reset the administrator password ***** booting into Mac OS X Utilities screen. Follow the steps below;
Turn off your Mac.
Turn it back on and immediately hold down Command and R keys at same time for 10 seconds.
This will boot your Mac to "Mac OS X Utilities” menu.
Click on the “Utilities” menu in top and select “Terminal".
Type (without space) : resetpassword
Press return. Confirm the user account and then change password ***** reboot as normal.
After restarting, if you get any message about Keychain then reset the keychain access database. Follow the steps below to reset Keychain;
Open Keychain Access, which is in the Utilities folder within the Applications folder.
From the Keychain Access menu, choose Preferences.
Click General, then click Reset My Default Keychain.
Authenticate with your account login password (the password ***** reset earlier).
Quit Keychain Access.
Please let me know if that didn't work, so we can continue troubleshooting.